Earlier quoted context omitted.
If he had done as you suggest, nothing would have happened, nobody would've noticed and he might have got $1000 in compensation if he was lucky. Instead, United look like a bunch of incompetent thugs and many of the people involved in this debacle will be worrying for their jobs, as they should. Laws/rules don't change if people don't make a fuss about them being wrong.
Right now he's looking at injuries and lack of a voucher/compensation, and possibly criminal charges. This situation is akin to getting kicked out of a store that you were asked to leave. Your rights and compensation are spelled out in the law dealing with situations like this. One person is not allowed to make these unilateral decisions because they feel as though the law is wrong in that time. Laws and rules don't…
I don't see United's contract of carriage rule 21 or rule 25 applicable. The passenger was granted boarding, not denied. FAR 91.3(a) doesn't apply, this has nothing to do with the operation of the aircraft. And federal regulation preempt state and local law, but you're welcome to try and convince me.
I see no contractual basis for involuntary removal after boarding is granted, other than being in breach of some other portion of the contract of carriage. The airline was basically in a position per their own contract, of auctioning off a seat on their own plane and they got pissy and called the police, which was improper, and then the police didn't bother to evaluate the appropriateness of removal.
Trespass doesn't apply, the passenger was authorized. Rescinding that state requires a basis in the contract, or FAA regulations or other law and so far the people engaging in victim blaming are providing a lot of jabber and no convincing explanations.