It was always the rule, but it might have changed. They did relax some of those tv rules a few years ago.
I think in the past if a doubleheader was on the other network, they would black out the game opposite the home teams timeslot. Idea was so home market would have to watch and or attend the home game to see football.
I think they did change that a few years ago. In the opener this year though it seems CBS does not have the national doubleheader since it is a 4:05 start, so obviously they would not show their one game at 1pm opposite the Eagles game in Philly.