Surely all the cable operators have deals in place with their preferred box providers, or is it more of a choose-your-own-box thing in the USA?
In the UK you order Virgin cable, it comes with a Virgin box. You order SKY intertubage and it comes with a SKY router, etc. SKY satellite is encrypted and hence uses the SKY box plus your card.
Unless it's a free-for-all in the US I don't see why it'd be in the cableco's interests to allow a third party access to the service feed, ultimately losing some control of the end to end provision ?