> someone else was getting away with a purse snatching, or beating up an ex-girlfriend, or playing the knock-out game, or emptying a cash register.

It doesn't work this way, unless it he tried his experiment at the busiest time of the day, and even then it is not 100% sure.

> The police are set upon from all sides.

We all are, we can all take the easy route by bending the rules, and risk getting in trouble.