You know, it occurs to me that part of their reluctance to jump in and help is that they know they have painted targets on their backs and that some people won't pass up an opportunity at a swing in the close confines of a crowd. They fear the people when outnumbered because all of their policies, strategies, and tactics involve controlling with fear. They don't see themselves as aiding fellow citizens, it's more like begrudgingly assisting the enemy you're actually supposed to eye suspiciously and subjugate. This feeds the cycle. Oh how they reap what they sow!
And to add: shooting a guy in an abuse of force during a traffic stop on an empty highway leaves no witnesses. A crowd will have a hundred eyewitnesses and dozens of phones recording.