Blah blah, Captain can throw off anyone they want blah Passenger was belligerent blah...
But isn't the problem (legally) that he was seated and not at check-in before they decided to bump him off the flight?
All the legal stuff about bumping passengers I've looked at refers to check-in and not whilst they are boarded. Considering he wasn't 'belligerent' or a 'threat to safety' before they decided to smash his face up and drag him unconscious down the isle, how can UA have a leg to stand on when this inevitably goes to court?