You stated "The owner of the plane can ask anyone to get off for any reason, as long as the plane is at the gate.".
I'm asking you for proof that it's legal to do....not anecdotal stories that it occurs.
Your airline ticket is a revocable contract. That’s well established. Of course there are good ways to revoke a contract and bad ways. In this case the timing was bad (who really wants to take the delay deal with their bags already on the plane and basically guaranteed to get lost), the reasoning was spurious, and the execution was terrible. But they can kick you off the plane if they want, it’s their plane.
Read it for yourself.
But in this case, the passenger was already onboard and the airline wanted to take him back off, presumably in order to put another passenger in his seat. Does United have the right to do that? Yes, because Rule 25(A)2(b) of United’s Contract of Carriage gives its boarding priority rules:
The priority of all other confirmed passengers may be determined based on a passenger’s fare class, itinerary, status of frequent flyer program membership, and the time in which the passenger presents him/herself for check-in without advanced seat assignment.
It doesn't say you get to act like a petulant child.