From the United CC:
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:
6. 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.
https://www.united.com/ual/en/us/fly/contract-of-carriage.html
That's just one little section. The entire contract makes it very clear that you are renting a seat for the duration of the flight when you buy a ticket.