Unzoned county?

It seems like a couple counties in my state are “unzoned” for outdoor advertising. Does this just mean that they have no county-specific rules and the state’s rules are the only ones that apply?

State zoning rules are the minimum, and county or city rules can only add more restrictions. Is this the correct way to think about it?

Almost all counties in the U.S. are “unzoned”, as you normally don’t have zoning unless you are within a city limits. In those counties is where the state definition of “unzoned commercial” kicks in, normally triggered by having two businesses side-by-side along the highway (but get your state’s specific definition).

You are correct that the Federal Act is the weakest, and then states, counties and cities can only add more restrictions.