Remember that the cables that they use were paid for with public tax money. The private companies are selling back to us the infrastructure that we already paid to build.
This is one issue on which neither progressives (who tend to favor government utilities/services) nor libertarians (who tend to favor the free market) are happy, because we have the worst of both worlds: a government-sanctioned, unregulated monopoly, using infrastructure built with public tax dollars, and re-sold to the original payers (the public) at monopolistic prices.
Unfortunately, recent efforts in this direction have completely failed. A North Carolina town dissatisfied with Time Warner banded together to create a municipal ISP, and Time Warner sued, on the basis that they "couldn't compete[0]" with the superior speeds and lower prices that the municipal ISP provided. In the end, Time Warner won[1].
I, like 80% of people in the US, have no choice as to my broadband provider. I don't have a landline. I don't have cable TV. All I need is Internet access, and for that, my only option is paying Time Warner Cable, at whatever price they decide to charge, for whatever service they decide to give me[2].
[0] read: "didn't want to compete" == "it's cheaper for us to sue you than to lower our prices to market rate"
[1] http://arstechnica.com/tech-policy/2010/11/the-price-of-muni...
[2] Pro-tip: If you're stuck with TWC as well, don't pay for the higher speed levels. That's just the maximum possible speed, and they don't guarantee you anything close to that, for any portion of your monthly service. If you live in a congested area, you're likely to get the same speed regardless of which tier you purchase, so just pick the cheapest one.