If anyone is interested in reading the United Airlines rules, they can be found at
https://www.united.com/web/en-US/content/contract-of-carriage.aspxTwo sections are of interest in this context
Rule 21 - Refusal of Transport
Rule 25 - Denied Boarding Compensation
Since the passenger was already seated, it appears that rule 21 applies.
There are lots of reasons why an airline can boot you off the flight. Too many in my opinion.
As for the police/security getting involved. If the airline crew decides that a passenger is in violation of rule 21 and "intimidating a flight crew member or flight attendant of the aircraft", the passenger may be in violation of 49 U.S.C. section 46504 which states
"An individual on an aircraft in the special aircraft jurisdiction of the United States who, by assaulting or intimidating a flight crew member or flight attendant of the aircraft, interferes with the performance of the duties of the member or attendant or lessens the ability of the member or attendant to perform those duties, or attempts or conspires to do such an act, shall be fined under title 18, imprisoned for not more than 20 years, or both. However, if a dangerous weapon is used in assaulting or intimidating the member or attendant, the individual shall be imprisoned for any term of years or for life."
Unfortunately, post 911, the threshold for 46504 violations has been relaxed. In any case, if the crew could reasonably feel that the passenger was in violation of 46504 (even if demonstrated that the passenger was not), the use of police force would be authorized.
It will be an interesting court case. I hope that they don't settle out of court. These types of cases need to go to trial. Things don't change when corporations settle out of court.