Well, the point is, United caused this themselves. The guy was a paid customer and was minding his own business, as he was let on the plane as a paid customer. And then they told him he won the lottery for being kicked off the plane, just because.
He then refused, saying he had to get home, so he could see his patients the next day. He did not care that they offered him $800.
Then they said, they'll forcibly remove him from the plane... which they did, and then some, bashing his head in the process.
Basically, he was a normal customer, until United screwed him over and threatened to throw him off the plane just because of their own issues.