Well every county has a local county cable commission (or something similar) who set the policies for the cable company operating in that particular county. I did a semester project at Comcast Montgomery County MD which is (or was at the time) a top 5 county for them. Every aspect of their business, from what the prices were to where call centers could be had to be approved by the local commission.

You might say that the county cable commission is corrupt but its probably no more so than the people who regulate power companies.

Not every county. It depends on the state.

Florida, for instance, is decided via the state, no local city or county has power at that level.

In other states, it can be set at the city level (different franchise agreements for each city without any county say).

Why are they a monopoly then? That was decided early on for most areas. The whole setup is designed to profit a few.