Re: Cable boxes are not "...only for rent."
It must be unique to your state. Here in California there's no such option. Comcast won't do anything other than rent you their equipment, and even if you buy a compatible device straight off the top of their list of them (a Motorola Arris, DocSys 3.0 in my case), they will lie out their arse & claim it's neither supported nor a DocSys 3.0 device. When I pointed out the fact that it was the *number one* device on their own compatibility list AND that the manufacture claims it's a DocSys 3.0 device, then and ONLY then did Comcast decide to pull their head out of their bum to let it connect to their network.
*Spits on Comcast in disgust*
You've probably also got the fabled thing called "Competition" in your state as well. It sure as fek doesn't exist in this one. =-(