They do -- not by banning it, but by enabling technology to filter it. The V-chip and cable/satellite parental controls allow parents (or just folks who don't want to stumble upon the nasty stuff) to filter what they don't want to see.
I'd like to see them take it a step further -- allow the customer, at request, to receive a cable converter that is restricted by default. Or, for that matter, set up the cable box with something like the old VCR+ codes, so that users could find an organization they trust and lock out a whole range of channels at one step. You could also work out a system for DVD players that lets you lock out films by MPAA rating (or, again, a third-party rating service).
I'm willing to accept broadcast as an exception to the first amendment for technical reasons, because bandwidth is scarce, and because only the government can effectively enforce the licenses. There is no such rationale for regulating print, cable, satellite, home video or the Internet. But the technology exists for the industry to regulate itself, and it will if consumers demand it.