Yes. The Pilot in Charge has the final say on who stays and who goes. Once the decision is made that you're off, you're off. If you won't leave you will be forcibly removed. If you resist with a weapon, even an improvised one, you're likely to end up dead.
It's akin to those people who say to the cops "Get your hands off me". You think the cops are just going to get in the cruiser and leave or do you think you'd be eating dirt in 10 seconds or less?
You surrender a bunch of rights when you step on that airplane and the pilot can kick you off for ANY reason. You'll have some recourse with the airline as far as payment goes, but the pilot's word is final.
If the passenger was breaking any laws he should have been arrested, but he was not arrested.
However, the police who dragged him off may have broken some laws and at the time the police were acting as agents of United Airlines.
United will have to write a check.
That's what a lot of people, including some Freepers, don't understand. It's called "Contract of Carriage." And the airline's right to remove a passenger is part of the contract.