An ethnicity is a completely different thing to a law enforcement agency.
edit: If you want me to explain why, I can. A law enforcement agency is a group of people who have decided to perform a particular duty. An ethnicity is a group of people who are born that particular way.
A law enforcement agency has a strict set of rules governing behaviour, which when broken purposely constitute corruption. An ethnicity does not a have a particular set of rules, they follow the same rules as wider society. If they did have a specific set of rules, it would be considered an apartheid style society.
A member of a law enforcement agency has far reaching powers beyond a typical citizen. A member of an ethnicity is supposed to have the same powers as everyone else.
What's interesting is that by comparing the two you get a good idea of why police corruption is a terrible thing.