Akamai and other CDN's do exactly this. They buy closet space on the providers networks so that they are extremely close by and on the customers network, not outside of it. Use anycast, and DNS that directs you to the closest site and you are no longer touching the public internet...

Akamai and other CDN's pay for this privilege to Comcast, and Comcast still counts those downloads against the users bandwidth cap...

That's an interesting way to look at it, but I would frame the issue in a different way. It's unlikely Comcast would pass on the savings to their customers. It's more likely that they would charge their customers more for accessing the other services that are not hosted in their closets. Then that becomes the scenario that so many people are worried about where big companies who can negotiate deals with Comcast have a huge competitive advantage over smaller competitors who might not be able to negotiate the same deals.