New York City police have been banned from all facial hair except for mustaches.
As wise man notes in the article:
"Appearance is so important in any kind of authoritative role," said Jim Curran, who was a cop in the 1960s and now teaches at the John Jay College of Criminal Justice. "Police officers should be clean. They should look nice. The Patrol Guide emphasizes that for a reason."
The best way to look nice? A mustache, of course!
It is sensible, though, to allow undercover officers to retain their various beards. They do mingle with outcasts and criminals of society and it would not do to look out-of-place.