I was recently surprised to see a man roughly and forcibly physically removed from an overbooked flight by three police officers because his flight was overbooked. He was reportedly a doctor who needed to see patients the next morning, so he did not really want to get off, justifying the use of force.
I knew that airlines could overbook flights and deny boarding to some passengers, but I thought that kind of boarding denial would happen at the gate and that once folks had been let on, those denied boarding would be those arriving later to the gate.
Why or in what conditions is this kind of forcible removal considered OK?
Clarification following close votes from people who think this question is primarily opinion-based: By "OK" I mean the sense of "legally OK," "officially accepted," "instantiated as policy," "sanctioned by authorities," or "OK according to the regulations and policies and laws and whatever other formal rules govern forced removal from flights due to overbooking." Whether or not you personally consider it socially acceptable does not matter as an answer, unless you have formalized that into a citeable adopted form that governs what happens and want to explain the reasoning behind that rule.