My $0.02
Having had a look at United's contract of carriage (https://www.united.com/web/en-US/content/contract-of-carriage.aspx?Mobile=1)
Section 25 clearly talks about denial of boarding, but this is irrelevant as the pax was already boarded.
Section 21 deals with refusal of transportation (offloading) but the pax does not meet any of the criteria.
Looks to me like the airline was not following it's own contact of carriage.
How does this affect those saying "it's United's private property and if he refuses to leave he is trespassing"?