A provider owned by a left-wing company could very easily slow down traffic on conservative sites. Freerepublic could see itself on the information slow lane. Or providers could allow sites they are affiliated with to have enough speed for HD broadcasts but keep others from being quick enough for streaming video.
A cable company, to site your example, could easily slow down traffic from services like streaming netflix, hulu, or youtube because those compete with their services. Comcast already tried this. A phone company could put the squeeze on Skype, etc.
One of the great things about the Internet and one of the things that has allowed it to become such a democratizing entity is the fact that the architecture of the Internet doesn’t allow it to become dominated by those with the most resources to the expense of everyone else. Its allowed an independent conservative media to spring up and has made it easier for sites like FR to come into being. Its also made it possible for things like Google to come out of nowhere. It would be much harder for the next Google, Facebook, etc to spring up in a world where websites from the big media and service companies or their affiliates were allowed to run faster than anybody else.
Your statements are all true ... and equally true is my ability to purchase internet access from different providers. If I dont like the way one company performs, I can take my business elsewhere. It is called the free market.