If you call the cops in the US for a passenger problem it should only be when they are a danger or a potential danger to themselves, the flight or other passengers,
Otherwise this is what happens, these cops have two modes, instructions followed or violence (perhaps death) used to achieve instructions.
Many cops relish the opportunity to use force, airport cops are pretty close to the bottom of the barrel in education and training, couldn't be 'real cops' so they're frustrated from sitting around watching TV, playing video games or whatever
This should have been handled entirely within the airline by a gate supervisor, its a disgrace what happened to this passenger.