PM, I fully understand and appreciate what you say. In fact you use the phrase "on a whim" which seems to apply to this case as there were many other options available that were either not considered, or considered and discounted. The path of least resistance would have appeared to be to remove the passenger who had already expressed a willingness to do so in the belief that a later flight was on offer; once it was clear this meant a flight the next day the circumstances for him changed and he rejected that offer.
Anyone who buys a ticket for an airline flight, has a boarding pass and is seated in a perfectly serviceable seat and doing no harm to anyone certainly has the right not to be assaulted; and subsequently humiliated and denigrated.
Unforeseen circumstances can occur and will continue to do so. These must be addressed on a case-by-case basis to ensure the safe conduct of the flight, and/ or the safety of the passenger(s). As you say, you have bought yourself out of those situations in the past; sadly in this instance UA closed the wallet too soon and it will cost them dearly. I am sure that for a few hundred dollars more someone else on that plane would have voluntarily relinquished their seat.