“

[T]he police do not and cannot protect law-abiding citizens from criminal violence. ... This thought may not occur to wealthy people who can shelter themselves in low-crime enclaves and who care not at all about their less fortunate neighbors. But no one knows it better than the police, who scrupulously preserve their own right to carry firearms on and off duty (and often after they retire as well) even while some of them advocate disarming those whom the police cannot protect.