One of the scariest subtleties with these situations is how police officers always chant "stop resisting" regardless of whether the person is resisting. It's almost as if they are explicitly trained to repeat that mantra. It's eery.
There's also of course, the conclusions about abuse of power drawn from the Stanford Prison Experiment: http://en.wikipedia.org/wiki/Stanford_prison_experiment
Either way, these cops should not be given such heavy weaponry.
They say "stop resisting" because saying it makes it harder to sue them for excessive use of force. Saying "stop resisting" is, of course, unassailable evidence of resistance.
As with many policing policies, crossing as many things off the cover-your-ass list at time of arrest seems to be very important.
Should there be a need to push 'resisting arrest' at a future date, it's probably better for someone to have said it than not. Even if this is not trained, it falls in line intuitively with policing attitudes towards risk of legal complications. It would be rewarded behavior that gets learned.
It's a terribly morbid version of soccer players taking falls. It's in their interest, as the rules of the game they are playing is written, to do so.