I think I finally figured it out

When Fox broadcasts the Super Bowl they advertise for their shows, like American Idol. But those years in which, say, ABC has the Super Bowl you will never see an ad for American Idol during the Super Bowl broadcast.

More generally, networks advertise their own shows on their own network but never pay to advertise their shows on other networks.  I never understood this.  But I think I finally figured it out, there’s some very simple economics behind this.