Jesus guys forget about the stupid "contract of carriage". We all know United can do whatever they want just like Wells Fargo can force all those customers who had accounts opened by their fraudulent actions could be forced into arbitration.
Ultimately if things like this continue to happen and be made a big deal out of (the kicker in this scenario is 1) the passenger was ALLOWED BACK ON and 2) he was black) and it costs United money things will change. Personally, I hope the guy sues United for all he can. He was let back on, that pretty much admits United screwed up and called the police on him for no reason.