I really have no idea how this tendency develops. But I'm sure what such cops are doing, it's contrary to their duty charter/profile - thus bad cops. Perhaps, some social scientist or behaviorist may put light on the issue. Since I'm an economist, I'd respectfully decline for passing on any comment upon behavior of such cops.
Really don't know if I've provided answer to your question or not haha.
Best Regards my friend,
Görlitz bhf (10)