Red jet and Icarus:
Had he got out of his seat and exited the aircraft as asked and as the other three passengers did then none of that would have occured.
Self inflicted.
Both of you make the case that the passenger was required by law to comply with what was purported to be a lawful instruction from aircrew (or cabin crew).
However the instruction itself was sufficiently bizarre enough for the passenger to challenge it and that triggered an overly aggressive response from the airport police.
I say "bizarre" because this type of behaviour is unheard of (until now) on an already loaded aircraft although it is not unknown before boarding. To put that another way, every passenger seated on that aircraft had a valid ticket and boarding pass and the
expectation that they would be carried to their destination, or as close as possible, etc. etc. United messed with passengers expectations and this is going to hurt them in future.
My experience with overbooking is that the standard tactic is either a hotel phone call or counter suggestion that "we can't find your name on the passenger list" followed by an offer to book you on the next available flight. The standard response to this, as my father taught me was; "Try someone else, here is my booking confirmation and ticket". I always carry documents in duplicate for just this reason.
I would have reacted exactly the same as the doctor in question - "try someone else".
The only sane responses to this situation would be
(a) Not overbooking - which is a disgraceful policy, visible at any American airport..
(b) Unloading all pax and then reboarding selected pax and compensating the others.
(c) Auctioning seats until enough people took the money.
As for the suggestion that the passenger should have complied with the instruction and then complained to ground staff, pull the other one. Once he was back in the terminal it's "mission is accomplished" for the airline staff. He can threaten to sue, but you and I know that the fine print on the ticket protects the carrier and he has a snowballs chance in hell of recompense.
All he would get is; "here is your new ticket and if you keep complaining I'll have you thrown out or arrested". There is no customer service in the USA, instead there are company peasants inflicting draconian corrupt company policy on customer peasants. That is why I'm never going back.