"The first digital video recorder (DVR) with a built-in commercial skipping feature was ReplayTV with its "4000 Series" and "5000 Series" units. In 2002 the main television networks and movie studios sued ReplayTV, claiming that skipping advertisements during replay violates copyright."
I think they do. And if it were as easy to strip ads from TV as it is for the web, you'd be hearing plenty about that too.