Not if they had the legal right to remove him.
It may have been legal, but there is an old saying; “You can be right, or you can be happy”. United chose “right”, and now they are not happy. Tough: They can just deal with it. My guess is that for the near future they will have some extra seats available. Just in case they need them for higher priority passengers.
Legal is often offensive to common sense,morality, and natural rights.
It’s a felony to threaten criminal action/consequences in a civil dispute. It’s also unlawful to commit battery on someone.
Still irrelevant to having him removed. He was not in commission of a crime while being in his seat. The crimes he’d committed were all in the past with no ongoing issue or penalties.