The monopoly is not entirely created by the government. It would be more accurate to say, "the major cable companies have divided the country amongst themselves and agreed not to step on each other's turf." The FCC and the local governments have various levels of complicity in this arrangement.
A municipality would not likely visit any violence upon a competitor, but that's largely because the only way to become a competitor (you can't just start digging holes and dropping cable in them) is through the approval of the municipality. This is where the "government-sponsored monopoly" idea comes into play, because there is often a revolving door between the government agency nominally charged with regulating cable service and the cable company that administers it.
Also, no, libertarians in general do not "believe in the concept of natural monopolies". Many libertarians hold that monopolies are short-lived and are prolonged only through the complicity of governments--or other actors which may play the same role in context, like the mafia, or even the corporation itself, should it become a de facto government. Of course, with that last point, you start begging the question...
And where municipalities have not received sufficient -- or any (i.e. high speed) -- service from existing providers and have therefore, finally, attempted to deploy their own local service (an action with much established precendent, e.g. rural electrification cooperatives, water service, etc. -- even telephone exchanges), in many of those cases the incumbent telecommunications providers have lobbied and coerced state governments to outlaw such actions.
They don't want to offer service. But they're damned sure not going to let anyone else do so -- not even let the community do so for themselves.
A municipality would not likely visit any violence upon a competitor, but that's largely because the only way to become a competitor (you can't just start digging holes and dropping cable in them) is through the approval of the municipality. This is where the "government-sponsored monopoly" idea comes into play, because there is often a revolving door between the government agency nominally charged with regulating cable service and the cable company that administers it.
Also, no, libertarians in general do not "believe in the concept of natural monopolies". Many libertarians hold that monopolies are short-lived and are prolonged only through the complicity of governments--or other actors which may play the same role in context, like the mafia, or even the corporation itself, should it become a de facto government. Of course, with that last point, you start begging the question...