All the television networks were ASKED not to broadcast the tapes and they agreed, for now. They were not ordered to and couldn't have been forced to comply if they had not wanted to.
As far as being an effective communication medium, it's perfect. Say that dozens or even hundreds of his cronies are in the US. How do you get a message to them all without arising suspicions? He tried satellite phones but the NSA was on to that. He could try the internet, but that leaves a path to follow as well. What if they were all told to watch the news and then perform their prearranged missions when, for example, OBL appeared in a tape wearing a certain outfit, or with particular people, or holding a specific item, or using a particular phrase. That's exactly the kind of trick he would love- use America's assets (i.e. technology) against her.