There is a lot of gun nuts in the United States. They are armed to the teeth and very delusional. With that in mind police officers should have the ability to meet force with force. This requires that they have AR-15's in their arsenal for dealing with the really crazy people.
The answer to law and justice issues is not "use a bigger gun." There is no practical reason why all cops would need to carry a rifle. An AR-15 makes much more sense for a SWAT officer or an officer in a combat situation. A beat cop with a gun like that makes no sense.