CGB
Yes, it is appropriate to overbook. Welcome to the airline world.
Bub
Don't equate the man being pulled off the plane as a failure of protocol. 3 of the 4 people to be removed did so, the fourth didn't comply with lawful order from the police officer and you saw the result. The aircraft isn't a public place, if those responsible and with the authority to remove the pax did so properly, then it's incumbent upon the pax to leave. The good Dr needs to remember he also has responsibilities as well.
Hems
Why would he get anything besides a refund? He didn't comply with a requirement to leave the aircraft from airline officials , and did the same to police. I have no idea if UA or the police have bigger plans for him, but covering his 3rd point of contact should be his concern, he's the only one who did anything wrong.