Replies

I think you should check the conditions of carriage. There is no rule giving United the right to remove a seated passenger because they want to give his seat to someone else. Lest you say he was causing a disturbance there was no disturbance until they tried to force him to leave the plane which according to their rules they have no right to do:
Rule 21 Refusal of Transport

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:

Breach of Contract of Carriage – Failure by Passenger to comply with the Rules of the Contract of Carriage.
Government Request, Regulations or Security Directives – Whenever such action is necessary to comply with any government regulation, Customs and Border Protection, government or airport security directive of any sort, or any governmental request for emergency transportation in connection with the national defense.
Force Majeure and Other Unforeseeable Conditions – Whenever such action is necessary or advisable by reason of weather or other conditions beyond UA’s control including, but not limited to, acts of God, force majeure, strikes, civil commotions, embargoes, wars, hostilities, terrorist activities, or disturbances, whether actual, threatened, or reported.
Search of Passenger or Property – Whenever a Passenger refuses to submit to electronic surveillance or to permit search of his/her person or property.
Proof of Identity – Whenever a Passenger refuses on request to produce identification satisfactory to UA or who presents a Ticket to board and whose identification does not match the name on the Ticket. UA shall have the right, but shall not be obligated, to require identification of persons purchasing tickets and/or presenting a ticket(s) for the purpose of boarding the aircraft.
Failure to Pay – Whenever a Passenger has not paid the appropriate fare for a Ticket, Baggage, or applicable service charges for services required for travel, has not paid an outstanding debt or Court judgment, or has not produced satisfactory proof to UA that the Passenger is an authorized non-revenue Passenger or has engaged in a prohibited practice as specified in Rule 6.
Across International Boundaries – Whenever a Passenger is traveling across any international boundary if:
The government required travel documents of such Passenger appear not to be in order according to UA’s reasonable belief; or
Such Passenger’s embarkation from, transit through, or entry into any country from, through, or to which such Passenger desires transportation would be unlawful or denied for any reason.
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:
Passengers whose conduct is disorderly, offensive, abusive, or violent;
Passengers who fail to comply with or interfere with the duties of the members of the flight crew, federal regulations, or security directives;
Passengers who assault any employee of UA, including the gate agents and flight crew, or any UA Passenger;
Passengers who, through and as a result of their conduct, cause a disturbance such that the captain or member of the cockpit crew must leave the cockpit in order to attend to the disturbance;
Passengers who are barefoot or not properly clothed;
Passengers who appear to be intoxicated or under the influence of drugs to a degree that the Passenger may endanger the Passenger or another Passenger or members of the crew (other than a qualified individual whose appearance or involuntary behavior may make them appear to be intoxicated or under the influence of drugs);
Passengers wearing or possessing on or about their person concealed or unconcealed deadly or dangerous weapons; provided, however, that UA will carry law enforcement personnel who meet the qualifications and conditions established in 49 C.F.R. §1544.219;
Passengers who are unwilling or unable to follow UA’s policy on smoking or use of other smokeless materials;
Unless they comply with Rule 6 I), Passengers who are unable to sit in a single seat with the seat belt properly secured, and/or are unable to put the seat’s armrests down when seated and remain seated with the armrest down for the entirety of the flight, and/or passengers who significantly encroach upon the adjoining passenger’s seat;
Passengers who are manacled or in the custody of law enforcement personnel;
Passengers who have resisted or may reasonably be believed to be capable of resisting custodial supervision;
Pregnant Passengers in their ninth month, unless such Passenger provides a doctor’s certificate dated no more than 72 hours prior to departure stating that the doctor has examined and found the Passenger to be physically fit for air travel to and from the destination requested on the date of the flight, and that the estimated date of delivery is after the date of the last flight;
Passengers who are incapable of completing a flight safely, without requiring extraordinary medical assistance during the flight, as well as Passengers who appear to have symptoms of or have a communicable disease or condition that could pose a direct threat to the health or safety of others on the flight, or who refuse a screening for such disease or condition. (NOTE: UA requires a medical certificate for Passengers who wish to travel under such circumstances. Visit UA’s website, united.com, for more information regarding UA’s requirements for medical certificates);
Passengers who fail to travel with the required safety assistant(s), advance notice and/or other safety requirements pursuant to Rules 14 and 15;
Passengers who do not qualify as acceptable Non-Ambulatory Passengers (see Rule 14);
Passengers who have or cause a malodorous condition (other than individuals qualifying as disabled);
Passengers whose physical or mental condition is such that, in United’s sole opinion, they are rendered or likely to be rendered incapable of comprehending or complying with safety instructions without the assistance of an escort. The escort must accompany the escorted passenger at all times; and
Unaccompanied passengers who are both blind and deaf, unless such passenger is able to communicate with representatives of UA by either physical, mechanical, electronic, or other means. Such passenger must inform UA of the method of communication to be used; and
Passengers who are unwilling to follow UA’s policy that prohibits voice calls after the aircraft doors have closed, while taxiing in preparation for takeoff, or while airborne.

https://www.united.com/web/en-US/content/contract-of-carriage.aspx#sec21