OK, it was legal prior to 1996 (was it the Cable Television Consumer Protection and Competition Act of 1992?), and majority of cable systems were established in the 20-30 years prior to 1996. So if a cable company was granted a monopoly by the government, and if it is able to keep competitors out because they enjoy monopolistic market share, isn't the critique fair?
If I want build a network in Comcast territory, there are very "new" customers. I have to go through an extremely expensive build out, and convince customers to switch. Meanwhile, Comcast can undercut me on price because they have already have infrastructure paid for. They can offer $1 internet to people trying to leave until I run out of money.
So yes, you are right that providers aren't currently monopolies, but they exist in markets distorted by being monopolies in the past.
edit: note that the Comcast/Time Warner mega providers grew themselves by buying up all the small ex-monopolies across the country - they didn't build out against them.