Fascinating that it happened to be a white doctor. Would it have played out differently if it was a pregnant female? Or a Muslim? Or a Mexican without papers? Fascinating.
Do the police get told how to do their job by the airlines? Interesting question. If the airport police are asked by the airline (or the Captain) to remove a passenger, then they will. Normally it will be because the passenger is drunk and disorderly, or has assaulted someone. On this occasion were the police advised that the passenger to be removed was a random selection? Or were they just asked to remove a passenger who refused to deboard.
Don't be too quick to judge the police. United has some serious explaining to do.