From a developer standpoint, I can't comment, since I'm not a developer.
However, if you think about it, these creatures pop into existence, more than likely in a scene of chaos. One of two things will happen, as in any situation of immediate reaction: fight or flight.
Now trying to figure out what these individual creatures are going to do, in each particular combat, is pretty widespread, and far too broad to come up with a definitive answer, so let's take the average "DM's discretion" situation as an example (and use one of my own personal experiences as well).
POOF! In the middle of a turmoil, a dark, dingy dungeon, full of orcs and goblins, a unicorn appears. Naturally, this completely random occurrence is going to confuse everybody, probably enough to stop the fight for a second. The unicorn, on the other hand, was just spending its evening getting ready for bed, and suddenly it's in this foul place!? How? Why? It's confused, and it's going to react.
Now, it sees two things: Orcs, and a group of people, mismatched but clearly not part of the orc horde. And one of them is hurt. As a creature of pure spirit, it would more than likely try to help those in pain rather than cause pain itself, so it opts to heal the injured man, then POOF! it once again vanishes, leaving a rather confused group of companions and orcs standing around, wondering what the hell just happened, until the barbarian realises he's still raging.
True, in this situation, a confused beast would likely react violently, charging at the first thing it sees, but not all creatures are dumb animals looking to shed blood. In these situations, they can either, stop, think, evaluate and react accordingly, or simply react out of fear and confusion, as is the same for any creature, person or deity in the D&D universe.