Except that is exactly how Verizon designed FIOS and AT&T designed U-verse. Both are delivered via IP, so what's the diff if you have a box provided by AT&T decoding an IP stream into CATV or if you have a box (such as the Roku) decoding an IP stream from NetFlix (and a lot of other optional channels)?
They charge you the rate they do based on expected network usage. If people start using the network differently then the system will bog down.
Here is a good analysis of the Netflix problem: http://market-ticker.org/akcs-www?singlepost=2226192