From a practical standpoint, a cop shouldn't wear long hair, because a criminal could grab him by the hair and pull him around. For the same reason, cops don't wear regular neckties.   They wear clip-on ties which will come off then pulled. At least there do where I live.

(I don't know how female cops with long hair deal with all this.)

For years my husband shopped for clip-on ties for cops at the local uniform store. He kept them on the back seat of the car and clipped one on at at the last possible moment before entering the office.