Answer:
The U.S Constitution doesn't say anything about the way local governments should be organized. Which means that it is up to each state organize itself.
According to historian David Barron there's an attempt of the 10th amendment referring to local governments but it is no, this amendment states "The power not delegated to the U.S by the constitution, neither denied by it to the states, are in charge of each state, or to the people " That last line is the one the historian misunderstands, but that is not enough argument.