My guess is that AA would assume that your main purpose is to get where you wanted to go when you wanted to get there. Certainly leaving you on the same flight (since it exists) makes more sense than rerouting just for COS. (And I mean that it makes more sense for normal people, if not for us, and their system is geared for "normals" since there are more of them). If the flight you were on had gone away, I would venture to say that they would have tried to find a suitable alternative with First available.

Cheers.