> You're invited to provide citations to the violated rules, regulations and laws.
From united contract of carriage:
UA shall have the right to refuse to transport or shall have the right to remove from the aircraft at any point, any Passenger for the following reasons...:
C) Force Majeure and Other Unforeseeable Conditions – Whenever such action is necessary or advisable by reason of weather or other conditions beyond UA’s control
H) Safety – Whenever refusal or removal of a Passenger may be necessary for the safety of such Passenger or other Passengers or members of the crew including, but not limited to: 3) Passengers who fail to comply with or interfere with the duties of the members of the flight crew.
He was definitely not complying with and interfering with the duties of the members of the flight crew.
Also, the distinction for boarding granting and non-granting is unclear to me. Is the fact that you receive a boarding pass indication that you're on the plane for good? Apparently not.
> people engaging in victim blaming
Assuming that the passenger was a victim. You should be blamed if you're at fault and unreasonable. If you're unhappy with the situation, deal with it later in the courts. Just like you can't talk your way out of a speeding ticket by "asserting your rights".