Bottom line is that it's United's aircraft. It would have been far better to deny boarding in the first place than to deboard a passenger, but the incident was escalated by the passenger refusing to leave the aircraft once told that he would not be accommodated on that flight.
I think it will set a terrible precedent if this passenger is rewarded for his behavior. The lesson learned will be that defiance of flight and ground crew and abusive behavior will get you want you want.