"belligerent demeanor it still does not give United Airlines the right to physically assault him. None of that is relevant to what took place."

First, POLICE assaulted him...  UAL didn't lay a finger on him.  

Second, belligerent demeanour will almost certainly escalate the level of physicality in encounters with the police.  If you disagree, try this... next time you are stopped at a RIDE check, when the officer engages you in conversation say "Let me through... I'm a Doctor." and refuse to get out of your vehicle when they insist (which they will).  The following minutes will look exactly like the footage from this flight.