The United States is different from other countries because our state and local law enforcement agencies are not nationalized/federalized. Therefore, no two departments are exactly alike. It's not hard to understand on the local level as most folks know who's what. I could see where a foreigner might be a little confused, but I think it's just another expression of our individualism that someone from another country might not understand. Jurisdictions here are also tighter than in other countries, thus different uniforms and cruisers are easier to understand locally. However, if it was ever codified I think a black and white theme for municipalities, green and white for counties, blue and white for states, and maybe pink and white for federal agencies might be easier to understand.