Not to fight over terminology, but in most of the US, cable companies are given an explicit monopoly "franchise" from the local municipality.

There are some areas that might be a natural monopoly anyway due to low density population, but only way you could know is open things up. That's a tough pill to swallow since the franchise "fees" (taxes) are guaranteed so long as the cable company has a guaranteed monopoly. Yes they could pass a generic tax on cable service, but that would be a new tax.

I focus on cable companies because that seems to be the most common way to get broadband to many parts of the US.