If a police officer issues you a lawful (to them) instruction you have to comply. Three other people did, no issue. He chose not to. His decision.
I would be pretty confident that the terms of carriage for his booking allow him to be offloaded.
The policeman (or whatever he was) should first have established that the captain had a lawful reason to have the passenger removed. You can't just say to a passing policeman "these people in front of me in the queue won't let me pass - please forcefully remove them!".
Do you know they did not do that?
And airlines will continue to overbook and undercater because some mathematician or accountant thinks its a great idea
Because passengers want low fares, remove overbooking and the price goes up.
If PDR is right then it was not "overbooking" in a usual sense, only that crew could not be accommodated. Three pax got off, this guy chose not to. Perhaps another pax should have been asked.