If a cop doesn't have a mustache, he's a rookie. There are no known exceptions to this rule. Cops are their own gang, and the cop stache is the "gang tattoo" they use. Independent style requires independent thought, and those with independent thought would never become a cop. The cop stache is an inevitability for any aspiring cop.
Police Chief: "You're no longer a rookie, Johnson. Grow a cop stache or get out of my jurisdiction."