Sorry, don't buy all this UA bashing. When you buy a ticket you agree to the airline's rules, which include overbooking (which besides keeping revenues up for the airline, also makes it easier to handle cancellations for less cost to the customer). This is not news, airlines have worked this way for years. Whether they needed the seats for other passengers or employees is irrelevant. And speaking of bad planning, when you know you have to be at work Monday morning and you schedule your return flight for Sunday evening...
I note that three other passenger were able to follow the rules. When this guy refused to leave and made the cops drag him out he was being an ass. He turned a civil matter into a criminal one. Sure UA could have tried to bribe him some more, but he agreed to the rules when he bought the ticket. Anybody can sue for anything, but I'll bet this does not go far. All he would be arguing is that he is entitled to more than he paid for.
See Rules 5 & 25: https://www.united.com/web/en-US/content/contract-of-carriage.aspx#sec5
You would not make a good lawyer...