I think yours is a flawed analogy, but I am not sure I can articulate why - but here goes:

Noone can force you to leave the Muslim or the black population; you are what you are, and so any suggestion that someone should be removed from that population is pointless (unless, of course, one thinks murder is an acceptable method; I am not suggesting that you do)

The police (or engineers; nurses; judges; etc) are a professional body; it is in their best interest that certain minimum standards apply to all members, thus making the public trust the competence of any representative.