It isn't just the haircut on its own. If there wasn't a history of this:

http://www.forgetthebox.net/wp-content/uploads/2013/08/Ameri...

Like say the police looked more like this:

http://cdn.caughtoffside.com/wp-content/uploads/2013/09/Swed...

Then haircut wouldn't matter. But I have yet to meet an American cop that didn't have a "military approved" haircut.