Because it was originally designed as an easy trap
That trap is incredibly dangerous for a party at the level the adventure is designed at (1st). But there are many things to consider on the adventure:
This allows you to heal two points of damage for every point of nonlethal damage taken. One of the pre-generated characters for this adventure is Kyra (cleric 1), which has 4 channel energy uses per day. This is an important fact, as once a character falls unconscious due to nonlethal damage, all nonlethal damage becomes lethal damage, potentially killing the party.
If a creature’s nonlethal damage is equal to his total maximum hit points (not his current hit points), all further nonlethal damage is treated as lethal damage.
At GM discretion, unconscious characters could become immune to the trap, as the trap is not aiming specifically at anyone and was originally designed (by the npc, not the adventure's author) to be loaded with real (lethal) arrows. They would simply wake up at a later time once the damage is naturally healed.
This trap was tailored specifically for this adventure, and there are some things the SRD does not mention:
- The SRD mentions that a tower shield can bypass the arrows, and anyone behind the tower shield is protected. There are three tower shields as loot a few rooms before, in the same dungeon. The adventure specifically mentions that people use them to bypass the trap, so I wouldn't be surprised if they had some arrows still stuck into them. Holding this cover is a standard action, meaning that they can still move and be completely protected from these arrows.
- While the arrows are being fired at the group, you can still make Disable Device checks (DC 20) to disable the trap. The pre-generated rogue has a +7 check and has thieve's tools (no penalties). Meaning that on the second round and up, she can try to disable the trap, which considerably lowers the damage output of the trap if she doesn't fail too many checks.
- The doors leading into the room with this trap are one-way doors, that will shut closed unless someone holds it open, and the trap activates as soon as someone opens the door into the room. This means that the room itself is a bad trap, as you could simply shut the door closed again and avoid it completely, or only one person could protect everyone else by holding a tower shield at the entrance, and so on.
- The average damage is sketchy here because, with +10 to hit, the trap has less than 50% chance to hit the group's fighter or rogue if they take the Total Cover action (if they decided to not use any tower shield). The total average damage would go down from 100 to 40-50.
With all that said: Yes, the CR for the trap alone is completely broken here. But you should consider where this trap is located as well.
Constructing the trap again
The trap building rules have nothing about nonlethal damage, but we could easily say that doing nonlethal damage reduces the CR by half, as it is hardly a threat. However, if used as written on the SRD, it is way more deadly than what is written on the adventure, especially if the creator designs it to wait for everyone to be inside the room, like triggering when they attempt to open the door to leave the room they are in, or in the middle of a long 10-feet (or 15 feet) wide corridor.
The original trap was designed so you could add any type of arrows inside (the villagers purposedly replaced them by blunt arrows), and thus the nonlethal part could really have been ignored when writing the SRD entry for this trap.
By the construction rules, here is the actual breakdown of this trap's CR if loaded with actual lethal arrows and properly located to maximize the damage output:
- Perception DC 20: +0
- Disable DC 20: +0
- Attack bonus +10: +0
- Average damage: +20 (4 avg per arrow, rounds to 10, multiply by 2 then by 10 rounds)
It's a CR 2 trap that is active by 10 rounds, making it a CR 20. Yeah, the building rules has a major problem when calculating the average damage, but that is what we got. Personally, I would call it a CR 8 trap based on the Challenge Rating rules for adding multiple monsters into the same encounter.
To build your encounter, simply add creatures, traps, and hazards whose combined XP does not exceed the total XP budget for your encounter.
Being a CR 2 trap, it should give you 600 XP. If we simply add 10 of these traps, that would mean 6000 XP and give us a rounded-down CR 8 encounter.
On the adventure, this trap is simply a fun encounter, not a real trap.