I'd like to believe what you claim is true, and it may be in part, but here in San Diego we have some major issues with some horrendous cops who have only been with the police department a few years. The biggest issue we're facing with our police department I can think of is sexual assault (and our former mayor resigned for that kind of behavior). In fact, our former police chief, just a few weeks ago, suddenly retired without explanation. The smartest thing our newly-elected mayor (a Republican, no less) did was to appoint a woman as police chief, the first female police chief in the city's history. Already, she's making some changes that I think will help.
But I digress - there are plenty of cops that are bad from the outset.