That's why overbooking exists... because usually someone like you will be happy to get bought out. Sometimes for a lot less than $800. In this case, nobody on the flight was biting, so United went deeper into the ticket contract that everyone agrees to without thinking about, and that's when people started wondering what their rights really were.
The reality is, your rights to stay on United's plane never really exist. If they ask you to leave, you have to leave (ticket or no) and dispute the costs later. They don't have to go over the contract with the police, all they have to do is say this passenger is being uncooperative with airline instructions (regardless of whether those instructions are to get naked, stand on one foot, and crack an egg on your head... or just get off the plane) and therefore the plane is unsafe to fly until that passenger is removed. The police will enforce that.
When the airline asks you to leave (or do anything) you know you're risking an encounter with the police when you refuse. That being said, portions of United's contract would likely be deemed unenforceable in court (for various reasons, some of which are based on assuming that passengers are morons), and damages will be awarded - or would have been, anyway, if he had left voluntarily. I don't know that United has to award damages for kicking off a passenger that won't comply with instructions.