Monopolies have always been a tricky mater when they deal with utilities because it's usually just a waste of money to have competing systems. For instance a small town is not going to have more then one power company because one power plant is much more efficient then two. But the towns power company is to be kept in line by the city council. Likewise many water utilities and garbage dumps are going to be run by private companies but not entirely free from local government control. Railroads were once in competition but became monopolistic due to the industries decline. But looking at comunications, there have always been monopolies, right back to the telegraphs and especially the phone companies. Bell was very shrewd in managing his technology to attract outside enterprise but still make it so everyone was dependent on him. For a long time, he got away with it, because Bell it allowed a nationally integrated system, which was a boom. Anti-trust suits were finally leveled when the nation was fully integrated. Same thing with microsoft, when a monopoly helped the public, no one cared about anti-trust. When microsoft tried to exploit their position, suddenly the justice department noticed the huge monopoly that had slipped past their radar for years. They could have run a monopoly forever if they didn't get too greedy. Google might be able to run a monopoly on web ads for decades, depending on how things go in the next couple years.
...wow, I got way off subject. But yeah, competition isn't really possible yet because the US lags so much in laying down broadband. So yeah, what they're doing is technically an illegal monopoly. But they won't get caught for it because there's little enough cable being laid down in this country as it is. Don't expect anti-trust suits anytime soon.