Earlier quoted context omitted.
They sell the upstream to hosting providers. http://en.wikipedia.org/wiki/Peering
I don't see how this is relevant. If Comcast were to sell this (or an equivalent device), they'd keep all the traffic within their own network.
When you have a peering contract, you offer your own network. Every bit not used is so valuable for cable providers, we consumers forget that because we all have flatrates.