Not all cops shave their heads. One reason is some dont have much hair anyway. Why do people in the military, and pro sports shave their heads. Many reasons not just one. Is it to look intimidating and discourage a possible confrontation maybe. Is it easier to care for? yes. You should ask the next officer you see.
There is a certain tactical advantage to a shaved head, or short hair. Control the head, control the person. Works with a thousand pound horse too. Hair makes for a great handle and control tool in a fight.
A little less fashion and a little more tactics.... PULEEEZE?