Denominator, it is wrong to blame United for something ALL airlines do with Department of Transportation approval.   You would not have known about it if the passenger did not resist being removed from the flight.  It is unfortunate that airport law enforcement officers had to be called to remove the passenger and that it got physical.   It is also wrong to blame United for that because the law enforcement officers work for the airport and not for individual airlines.  Having said that, overbooking and forced bumping is a problem that needs to be addressed by the US Department of Transportation and by Congress, if necessary.