I would actually put the least blame on the law enforcement officers; I'm sure they were just called & told an unruly passenger needed to be removed or something of the like. We all have seen enough videos of insane passengers who need to be forcefully removed for the protection of other flyers that police (unfortunately) believed united that this passengers removal was a necessity.

All the blame falls on United for their poor handling of the situation (from the seating of the passengers, to offering low compensation for volunteers, to threatening to not take off unless people volunteered & finally calling for the forceful removal of passengers)