They don't. Perhaps if there is an off-duty, non-uniformed cop right next to an armed robber as he is trying to rob a convenience store, he might take action.
Might.
However, look at the Robb Elementary School Shooting. Cops were on the scene fairly quickly, but they failed to act. You can watch the video, and they were all scared of this lone, inexperienced, child-like shooter. Any one of them could have taken the guy out, but they just didn't. They were protecting their own asses.
Some cops investigate crimes after they are committed, if we taxpayers are lucky. But, most are just "out there" for appearance. Police Departments and the FBI have become places to stuff fat, lazy, out-of-shape, average IQ, people who can't make a go of it in the private sector.
The police are there to collect their paycheck, give out some traffic tickets (
makes it look like they are doing something in monthly reports), get great health benefits for their families and possibly a pension. They are not there to protect you, your kids, your family -
that's all ****!!!