Something very strange about this story. Up here in mild mannered Canada we have a simple process for over booking that is basically free for the airlines.
The gate agent makes an announcment asking for a certain number of passengers to give up their seats. In exchange they will be put on the next flight in business class. If no one goes for this they throw in a $100 voucher. If still no takers the voucher goes up until the require number of passengers agree to get off. Normally just the upgrade is enough.
To haul a cutomer off in a violent manner is really unbelievable but the problem is the United CEO. If he was effective he would say that this was unacceptable, make it up to the passenger and ensure that all United employees are properly trained. Fat chance!!