From the BBC News website...
Can an airline really treat passengers like this? - by Simon Calder, travel correspondent for the Independent
Yes. The captain is in charge of the aircraft. And if he or she decides that someone needs to be offloaded, that command has to be obeyed. From the moment that the unfortunate individual in this case said, "I'm staying put", he became a disruptive passenger.
From that moment he was disobeying the captain's command. Officials were legally entitled to remove him, and as the videos show, he was dragged from the plane. It appears from the evidence that the law was broken - by him, not by the airline. But I would be surprised if United pressed charges.