While I am not condoning the actions of the "police", UA does have a clear policy on over booking.
Under to United's
Contract of Carriage (or rules for passengers), "If a flight is oversold, no one may be denied boarding against his/her will until UA or other carrier personnel first ask for volunteers who will give up their reservations willingly in exchange for compensation as determined by UA. If there are not enough volunteers, other passengers may be denied boarding involuntarily in accordance with UA’s boarding priority."
The policy was followed properly with initially a $400 plus hotel stay to take a flight the following day which was later bumped to $800. When not enough volunteers took the offer, passengers were then voluntold. The problem isn't the policy, it is the action taken by security when the passenger did not comply with the request.