The FCC rules do not say anything about retransmission of the national network feed. The DMA is setup so the broadacster serves the community and as such the FCC only cares about the local content that the broadcaster generates.
said by cramer:
...otherwise DISH and DTV would simply carry 6 national broadcasts and drop the thousands of local stations -- and the billion dollar satellites they built to broadcast them.
Dish/Direct don't do this because ABC, NBC, CBS, etc won't allow it because it would harm their relationship with the local affiliates.
However, the difference in this case is FOX has essentially dropped the gauntlet and acknowledged that they really don't need their affiliates as much as they did in the past... and I wouldn't put it pas NBC, CBS, etc to do the same in the near future.