When children are young, you narrow down the media that they have access to seeing, and you build in them the understanding of what is "bad" and what is "good". As they become older, the parent, knowing that they can never control fully wht the child will be exposed to, allows them greater and greater access to the boatload of crap that is out there. I dont' mean you put it on their plate, but you dont' say "No" when they ask to see a violent movie, or one with cuss words. If you have actually educated your child, they will be able to see that stuff and know it for what it is. Crap. (I am thinking of the above example of Jersey Shore). Or, better yet, once they have seen it a few times, they will lose interest.
If the parent disallows teens to see any of the media that they know is bad, then the teen will only want to see it more. Forbidden fruit looks much tastier than it really is.