[lutorm]

As I understand it, it's unclear whether they followed procedures and the law. The law and procedures governing oversales allow "involuntarily denied boarding" if there aren't enough seats. However, a completely different section of the contract governs the circumstances under which a passenger may be removed from a plane, and that basically comes down to safety.

Some people argue that "boarding" continues until the plane is in the air and that the airline can execute their right to involuntarily "denying boarding" to any passenger up to that point. Others argue that the plain meaning of the word means that once you are "aboard" the aircraft, you can no longer (legally) deny someone boarding. They argue that since the contract of carriage does not define "boarding", it must be construed to have the plain language meaning (as in the United CEO in his statement saying that "the flight was fully boarded".)

Airlines may do it all the time but, as far as I can tell, a court has never ruled on when "denying boarding" ends and "removal from aircraft" starts. Maybe now we will find out.