The ticket he bought entitled him to have UA haul his butt from one place to another. Period.
It was not a guarantee that he would have THAT SPECIFIC SEAT on THAT SPECIFIC PLANE.
And if he had simply stayed off the plane after he had been removed, UA would have given him a ticket on another flight and sent him on his way: Since the SOLE obligation of UA was the get him to his destination, once he arrived, UA's part of the contract was fulfilled.