It's not just the NBC money, Notre Dame also has the cushiest bowl deal in college football. They'd be foolish to throw that away as long as there are conferences like the ACC willing to accede to them in some far-fetched hope of someday getting full conference membership.
ND easily got the better end of this deal, setting themselves up for the post-BCS era and all it cost them was affiliation with their sports that don't matter. The ACC just gave up a bowl bid for a few non-conference games, half of which they won't even get adequate TV revenue for.