I can't see how United can be in the right here. Yes they have certain rights to remove passengers for cause, but that cause should be limited to those passengers that are disruptive, cause a threat, or are otherwise not behaving properly.

To physically remove a boarded, revenue paying passenger sitting in his proper seat is unacceptable, especially when it is so the airline can board additional airline employees. The airline runs a buiness and as others have said, they should not have boarded if they were short on seats. Rather they should have made increasing offers until they got the 4 seats.

I am not an attorney, but that said: Once passengers boarded, the airline has redeemed their ticket for passage to their destination, and that passenger has every right to remain on the plane. Kicking someone off because the airline needs the seats should never be allowed, much less physically removing a passenger who isn't causing a threat to the crew or other passengers.

United clearly overstepped their bounds here and deserves not only the bad press, but also their behaviors should be reviewed by the FAA.