While I won't dispute that the moment he refused to deplane, he violated the law (you're required to follow the instructions of flight crew), the request that he be removed was also a violation of UA policy from what I've seen.
Here's some things that I've read
- He had previously volunteered to deboard, until he learned that it would be such a long delay
- This was not a case of overbooking, the 4 people added to the flight were United Employees, not overbooked passengers.
- UA policy states that airline employees will be the first people bumped from a flight in the case that it is overbooked
- (According to someone on reddit, so extra salt:) All of the overbooking regulations have to do with seat assignments. You shouldn't get bumped once you're assigned a seat, much less seated, and doing so may be a breach of regulations by United.
If that's the case, then the crew were absolutely breaching UA policy when he was removed, and they may have been breaching federal regulations when they asked him to deplane. To be clear, that's no excuse for refusing to deplane (even if he knew he was in the right), but it's not as simple as "he was disruptive" if the request for him to leave was illegal.