The owner of the plane can ask anyone to get off for any reason, as long as the plane is at the gate.
Kindly point us to the document that states that specifically.
According to their own contract, the passenger only could have been denied boarding the plane.
http://www.dorfonlaw.org/2017/04/united-airlines-own-contract-denied-it.html
The contracts states they may be removed for a list of specific reasons such as disorderly conduct. Nowhere is it ever even implied they may be removed for ANY reason.
“The owner of the plane can ask anyone to get off for any reason, as long as the plane is at the gate.”
It has to have reasons that meet the law and its own terms of service. It cannot be merely at the whim of the airline, any airline staff, or the captain of the plane. That does not mean they would be unsuccessful at removing you, beyond the law and their own terms of service, but if they do they CAN be held liable for WRONGLY removing you.
The owner of the plane?
Most planes are leased, the airlines don’t buy them.
http://www.nytimes.com/2007/05/10/business/10flyboy.html