Why do they have to make sure Comcast doesn't dominate over competing companies?
Because now that Comcast owns a company that provides content delivered on their service. The more profitable their subsidiary company is, the more profitable they are.
Say you have Comcast internet, and you're thinking about dropping your Comcast cable because Netflix is a lot cheaper and way more convenient. Well, suddenly, Netflix runs a lot slower than other sites! Suddenly, it takes just as long to load a show as it does to watch it! Unacceptable; I guess you'll just keep your Comcast cable and cancel your Netflix.
Shouldn't be much of a stretch.