I think people are mad at him for a few reasons, one being that it was rumored LSU was after him and every time he would argue that he is the coach at OU and thats it. So regardless of how it went down it was brought up many times and Riley wasn't forthcoming.The problem with that is that Riley was being called a snake before even any rumored details came out. In fact, before it was officially announced that he was leaving. So I don't think it had anything to do with the way he did it (in many fans minds), as there was no information whatsoever about how it might have gone down.
Some of the stories that have come out since don't sound very good, but as noted above it's very one sided.
Also, I don't think too many people are upset with Riley alone leaving, its leaving before the bowl game, taking many of the other coaches with him, and then the de-committing from the multiple recruits last night and today.