What stops the 6th-level Clr spell animate objects [trans] (PH 199) from making corpses dance is that
Corpses Aren't Objects
The game defines creature as a "living or otherwise active being, not an object" (PH 306). The game defines active only with regard to spells (e.g. the skill Concentration (PH 69-70)), but this is also addressed below. The game defines object as "[a]nything that lacks [i.e. has as a nonability] Wisdom or Charisma..., not a creature (though it may be alive, such as a tree or a sponge)" (MM 298). The game defines dead like this
A character dies when his or her hit points drop to –10 or lower. A character also dies when his or her Constitution drops to 0, and certain spells or effects (such as failing a Fortitude save against massive damage) can also kill a character outright. Death causes the character’s soul to leave the body and journey to an Outer Plane. Dead characters cannot benefit from normal or magical healing, but they can be restored to life via magic. A dead body decays normally unless magically preserved, but magic that restores a dead character to life also restores the body either to full health or to its condition at the time of death (depending on the spell or device). (PH 307)
At no time does the condition dead say that a creature who gains that condition becomes an object. At no time does the dead condition say the creature who gains that condition loses his ability scores, turning them into nonabilities, therefore making the creature into an object.
The (object) tag on a spell's saving throw or spell resistance entry means that the spell
can be cast on objects, which receive saving throws only if they are magical or if they are attended (held, worn, grasped, or the like) by a creature resisting the spell, in which case the object uses the creature’s saving throw bonus unless its own bonus is greater. (This notation does not mean that a spell can be cast only on objects. Some spells of this sort can be cast on creatures or objects.) A magic item’s saving throw bonuses are each equal to 2 + one-half the item’s caster level. [...] The term... "object" mean[s] the same thing for spell resistance as [it does] for saving throws. (PH 177)
The following Player's Handbook spells interact with corpses specifically.
- the 5th-level Sor/Wiz spell animate dead [necro] (PH 198-9) says, "This spell turns the bones or bodies of dead creatures into undead skeletons or zombies." Further, it has the entry Targets: One or more corpses touched and has no entries noted with the (object) tag.
- the 6th-level Sor/Wiz spell create undead et. al. [necro] (PH 215) says, "The spell must be cast on a dead body." Further, it has the entry Target: One corpse and has no entries noted with the (object) tag.
- the 3rd-level Sor/Wiz spell gentle repose [necro] (PH 235) says, "You preserve the remains of a dead creature so that they do not decay. Doing so effectively extends the time limit on raising that creature from the dead." See below.
- the 9th-level Sor/Wiz spell soul bind [necro] (PH 281) says, "You draw the soul from a newly dead body and imprison it in a black sapphire gem. The subject must have been dead no more than 1 round per caster level." Although it has the entry Target: Corpse, the spell soul bind allows the dead creature a Willpower saving throw and lacks an (object) tag.
- The 3rd-level Clr spell speak with dead [necro] (PH 281) says, "You grant the semblance of life and intellect to a corpse, allowing it to answer several questions that you put to it.... You can cast this spell on a corpse that has been deceased for any amount of time, but the body must be mostly intact to be able to respond." The spell speak with dead further allows the dead creature a Willpower saving throw and has no entries noted with the (object) tag.
- The 6th-level spell stone to flesh [trans] (PH 285) says, "For example, this spell would turn a stone golem into a flesh golem, but an ordinary statue would become a corpse." A unique case of corpse being used in an example.
However, there are several not easily resolved inconsistencies with this.
The spell gentle repose has the entries Saving Throw: Will negates (object) and Spell Resistance: Yes (object), but Target: Corpse touched. Only this one spell meets the requirements for proving that corpses are objects because the spell can't be cast on anything but a corpse. However, if that's proof that that corpses are objects, why does the game believe that corpses should get saving throws versus being merely preserved but not versus being animated or turned into created undead? It's odd and should either be house ruled as...
- corpses are creatures and the spell gentle repose should not have the (object) tag in its saving throw and spell resistance entries.
- corpses are objects and the spells animate dead and create undead et. al. should have the entries Saving Throw: Will negates (object) and Spell Resistance: Yes (object) instead of Saving Throw: None and Spell Resistance: No, unless there's a compelling reason the standard Dungeons and Dragons 3.5 campaign discourages corpse preservation while encouraging corpse animation.
There's that word active in the PH definition of creature; to reiterate, a creature is a "living or otherwise active being, not an object" (306). The word active is used throughout the rest of the game with regard to spells, therefore there's no way to understand this use of the word active except as an isolated, unique use.
There are many ways in Dungeons and Dragons 3.5 to make a creature not active if active is given a broad definition. If not active means unable to take actions, creatures that are cowering, dazed, dying, fascinated, helpless, paralyzed, stunned, or unconscious--in addition to being dead, obviously--are not active. Because the dead condition is only one thing of thousands (given all the game's magic items, spells, and special abilities) that can make a creature not active, the active part of the definition of creature must be narrower than unable to take actions. For example, if active means unable to take actions, then stunned creatures become objects.
In the definition of creature, then, this unique use of the word active is meant only to include creatures that aren't living--constructs and undead--as creatures, too. The word active isn't being used as a commentary on the creature's current ability to take actions.
The magic item unguent of timelessness (DMG 268) (150 gp; 0 lbs.) says
When applied to any matter that was once alive (leather, leaves, paper, wood, dead flesh, and so on), this ointment allows that substance to resist the passage of time. Each year of actual time affects the substance as if only a day had passed. The coated object gains a +1 resistance bonus on all saving throws. The unguent never wears off, although it can be magically removed (by dispelling the effect, for instance). One flask contains enough material to coat eight Medium or smaller objects. A Large object counts as two Medium objects, and a Huge object counts as two Large objects.
But the unguent works specifically on dead flesh, not corpses, and because this whole thing is hair-splitting, I'm going to do it--pieces can be sliced from folks--alive or dead--and those pieces will be objects, but the complete corpse isn't an object. Yeah, I know, I don't like it either, especially as oil of gentle repose isn't an thing that can be rolled, but there it is. An unguent of timelessness can totally preserve an arm or head but not a corpse.
Objects are defined by having Wisdom and Charisma as nonabilities. The Wisdom nonability reads
Any creature that can perceive its environment in any fashion has at least 1 point of Wisdom. Anything with no Wisdom score is an object, not a creature. Anything without a Wisdom score also has no Charisma score. (MM 313)
And the Charisma nonability reads
Any creature capable of telling the difference between itself and things that are not itself has at least 1 point of Charisma. Anything with no Charisma score is an object, not a creature. Anything without a Charisma score also has no Wisdom score. (MM 313)
However, it's overreaching to assume the inverse.
The game doesn't say that
- Any creature that can't perceive its environment in any fashion has Wisdom as a nonability.
The game also doesn't say that
- Any creature incapable of telling the difference between itself and things that are not itself has Charisma as a nonability.
Creatures who are, for example, merely unconscious don't become objects, yet those creatures can neither perceive nor differentiate between themselves and other things. Although there are vast differences between unconscious and dead, both negate a creature's perception and capacity for differentiation. Just because one effect usually lasts longer doesn't gives another effect greater weight.
Other, later texts might attempt to override the Player's Handbook, Dungeon Master's Guide, and Monster Manual on these points, but according to the core rules, that's how corpses work.
"But Why Does the Game Want Corpses to Still Be Creatures?"
Creatures don't become objects when they're dead because many spells affect creatures whether they're dead or alive. Making dead creatures objects forces a massive, detail-heavy overhaul of many, many spells with extra conditional cases for a niche part of the game, greatly adding to an already lengthy portion of the text. Keeping dead creatures as creatures, while counter-intuitive, is consistent with creatures retaining value even when they gain the dead condition and permits easier writing of later spells.
The following examples illustrate the subtle changes that happen in the game if corpses are considered objects instead of creatures. For some, these illustrations might encourage the view that that corpses aren't creatures and instead objects, and to those that it does, please read on, and remember to consider carefully every spell and power that targets or incorporates an object can now target or incorporate a corpse, too.
The 0th-level Sor/Wiz spell light [evoc] (PH 248) has the entry Target: Object touched. If dead creatures are still creatures such a spell can't be used to make creatures glow, but if dead creatures are objects, then dead creatures can be made to glow. A wand of light can now be used to locate a vampire who thought himself safely concealed among the dead in a morgue to where the spell light couldn't've done that otherwise. Should the spell light penetrate disguises?
The 8th-level Sor/Wiz spell trap the soul [conj] (PH 295) allows the use of a trigger object and if the caster "tricks the subject into accepting the trigger object inscribed with the final spell word" the creature's soul's transferred into a specially prepared gem. If a dead creature's still a creature, the dead creature can't be used as a trigger object. If dead creatures are objects, the evil conjurer can offer an owner's dead animal companion's body to his owner, and, when the owner takes the inscribed animal companion, the conjurer steals the owner's soul. Should corpses carry such risks?
The 5th-level Sor/Wiz spell permanency [univ] (PH 259-60) can make a spell permanent on an object that can't be made permanent on a creature. However, if a dead creature is an object the dead creature can be the subject of spells made permanent and then returned from the dead, permanent spells intact. While this is interesting ("Hey, while you were dead, Gazoo put a permanent symbol of death on your forehead--here's your hat--and a permanent teleportation circle on your back; now take off your shirt and lie down so I can get back to town and cash in these gems"), that's clearly not the intent. Should death allow the opportunity to have spells made permanent on one's dead body that's later returned from the dead that are impossible to make permanent on one's living body?
Consider the possible effects these spells have on corpses: the 2nd-level Sor/Wiz spell shatter [evoc] (PH 278), the 4th-level Sor/Wiz spell fire trap [abjur] (PH 231) (the spell "creates a fiery explosion when an intruder opens the [object]"), the 9th-level Sor/Wiz spell refuge [conj] (PH 269-70) (to cause the magic to happen "the subject rends or breaks the [object]"), the 2nd-level Clr spell make whole [trans] (PH 252), and the 2nd-level seer power object reading (XPH 123).
Ironically, This Is Not a Hill to Die On
As an aside, I have no dog in this hunt, no personal investment at all (except having written a really long answer). If the DM's already ruled that corpses are objects in his campaign, that's great, and the game will continue functioning. It changes things only a tiny amount. It really is a niche rule that rarely comes up, but it probably will come up at least once per campaign, and it won't if creatures aren't objects.