^ ^ I don't think they're only allowed to get video from just NBC affiliates.  They tend to take more live looks (weather cams) from NBC affils, but I think the video can come from all over.  As long as it's not embargoed to them and they get it via another news service.  But, they have to be a part of that service, as well as the other station they're "cherry picking" from.  (Example: NNS is a news service.  If TWC is a member, they can use video from ANY other station that's also a member of NNS, no matter its affiliation.)  I'm not explaining this very well, so I hope that makes sense...
Also, I work for an NBC affiliate, and during the Florida Freeze, we actually used a good deal of video from them.  Kind of sad, though, because I work in Florida...