Police become and stay corrupt because they know they are protected. "Thin Blue Line" and "Blue Wall of Silence" type stuff. Police unions really perpetuate this type of behavior by discouraging actual scrutiny of charges brought against officers. Police unions take a "we'll get you out of this, here's what you say and do" attitude taken towards charges all charges. And if they can't get all the charges outright dropped, the punishment is usually a slap on the wrist.
And don't get me started about "administrative paid leave".