"Police can't normally walk away from the scene, and they are compelled to attend in the first place."
Uhhhhh - no. The police are not obligated to come to your aid. Never have been, never will be. When you call the police, they only come if it is convenient, and when they feel like it.
http://www.nytimes.com/2005/06...
As for walking away from the scene - I'm not aware of anything that binds any individual officer to the scene. He may walk away from a confrontation at any time.
There are some pretty good discussions going on right now about such things.
http://www.policemag.com/chann...
In short, a cop can decide to defuse a situation by just backing off, at any time he chooses to do so. And, in fact, some of the wiser heads in the various police forces say that they should do just that. Not always, but often.
How many stories have we read of, where some mentally deficient person was shot to death, simply because he wasn't cooperating? And - the cop feels "threatened". One of the most recent stories I remember involved a nut case who was on his own porch, and happened to have a screw driver in his hand. There was no indication that he intended to use that screwdriver as a weapon - the cop just "felt threatened" because of that dumbass 21 foot rule. Yet another dead nutcase - and no one answers for the killing.