Nothing in any federal or state statute or regulation authorizes an airline to remove anyone from any aircraft to accommodate deadheading (traveling for free) crew members. If the forcible removal was legal, it would have to have been authorized by the relevant contract of carriage, which binds the passenger regardless of whether he or she gets a copy of the contract or even a notice of its existence. U.S. DOT regulations require carriers to give passengers written notice about the existence of the contract of carriage, and the carriers typically do so when the GDS issues an e-ticket with a lot of fine print under the heading, "Notice of incorporated terms of contract."
United's contract of carriage has no less than 26 grounds for removing passengers from aircraft, but not one of them states that a passenger can be removed to accommodate a deadheading crew member. The closest that United comes is the grounds called "Force majeure and other unforeseeable conditions." However, United has not cited any unforeseeable condition in this case.
This means that United breached its contract with the passenger, who can now sue for the breach. Because of the federal preemption rule, he may have trouble suing on other grounds that could lead to higher damages, such as intentional infliction of emotional distress or false imprisonment.
For the future, United could amend its contract to include the crew needs as a 27th ground for removal, in which case the passenger could not even sue for breach of contract.
Numerous articles and social media commentaries referred to the DOT's Denied Boarding Compensation regulations, which provide that involuntarily bumped passengers are entitled to collect up to $1,350. However, those rules apply only to "oversales," when too many people have confirmed reservations. In this case, there apparently was no oversale so United could not justify its actions by citing those rules. In any event, I don't think that the denied boarding rules allow removal from the aircraft as opposed to denial of boarding in the first place. Although United did allegedly ask for volunteers in return for money and only chose to remove the passenger after there were not enough volunteers, use of this procedure did not excuse United's breach of the contract.
What you don't seem to understand is 'Force Majeure' does not mean "We can legally do what we like when we feel like it." there has to be a justified reason. "After boarding we forced you off to make room for our staff" doesn't qualify.
He literally mentions force majeure as the reason they would use, but dismisses it because at the time they hadn't cited a reason for it. Since that time they have cited a reason which is that they had a crew issue at the destination airport caused by weather and government regulation, both force majuere reasons. ---