I can only assume that most airlines frown on people paying for economy class then simply taking a first class seat because it was empty. While I still don't understand how this guy was not caught prior to departure, I am glad that AA took it seriously and dealt with it accordingly. I either buy first class tickets or upgrade through airline approved methods, so somebody taking the upgrade otherwise is nothing more than theft of services in my opinion.

I am not suggesting the person mentioned by the OP was a self upgrader, but merely that I had a similar experience with police escorting a passenger off a flight.