Also, counties often have their own sheriffs with a separate set of powers and responsibilities. Sometimes court systems have their own enforcement arms, separate from other agencies.

The US often seems to have more layers of administration and law-making than some other places. Making individual officers work through which set of powers and laws they're dealing with in a given moment seems like it might at times be a daunting prospect. You could in theory organize them into a single bureau, but I suspect you'd inevitably wind up with specialized sections to deal with the particular laws in given cities.