Yeah I've noticed that too. I wonder if it's a safety precaution they had to implement to keep the other cars from taking each other out
I remember watching a video that the Forza team put out maybe a half year ago that was talking about all of the "Great Stuff" they were putting in the game and they were talking about debugging the AI - one of the things the guy in the video mentioned was watching the AI race and thinking "now why in the hell did he do that?"
I've had a few of those moments myself. One time on the New York City track, at that first real turn after the loooong straight one of my competitors took the turn too tight and hit the wall. Then on my next lap, he was still there, just backing up, and then going forward into the wall again. I think he finally got out of that jam around lap 5 or so. It was pretty funny.