Bumping prior to boarding is indeed a long accepted practice. Removing someone from a flight after boarding is not. It is counter to the law except in the rare circumstance that the person poses a risk to the flight.
Correct. He shouldn’t have been removed. However, the airlines failure to follow regulatory requirements in this case is not a free pass to trespass or interfere with a flight crew.
https://www.transportation.gov/individuals/aviation-consumer...