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.
Looking at what we know at the moment, the jury is still out on a number of assumptions you have made. For instance the Department of Transport rules say:
DOT requires each airline to give all passengers who are bumped involuntarily a written statement describing their rights and explaining how the carrier decides who gets on an oversold flight and who doesn't.
Equally:
DOT rules require airlines to seek out people who are willing to give up their seats for compensation before bumping anyone involuntarily. Here's how this works. At the check-in or boarding area, airline employees will look for volunteers when it appears that the flight has been oversold.
What marks this case out is that the pax was already on the plane and had a seat. So he was being forceably removed to accommodate someone else.
UA is going to have to provide some really strong reasons, in accordance with the policy, the DoT mandatorily requires, to justify their action.
We all wait with baited breath to hear their justification.