All I can say, after 40 years in the professional aviation business, is that I'm appalled by these UA thugs.
The man bought a ticket, paid for up front. As far as I can see, it's a legally binding contract. He saw his need to be on that particular flight as greater than that of the airline staff, who could have used other means. For him to be assaulted in that way was totally uncalled for.
United Airlines? They can stuff it.