censorship and rating systems

I know that some of the stories I am writing are not for the faint of heart,
but what I want to know is why is there censorship on T.V.,
and why do movies and television have a ratings system while most books do not?

I know that there are many books for children, and teens like J.K. Rowlands' Harry Potter series. But when I was under eighteen I could buy a Stephen King novel and not get the third degree from the sales clerk.

What do you think?
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