I am especially confused on how arbitrarily the censorship is applied. I could maybe understand (but not be in favor of, of course) a law that made depicting any crime illegal. But this will obviously never happen; fantasizing about murder and terrorism and depicting it in great detail is a recipe for a blockbuster film. As soon as it's having sex with someone below a certain age, though... then you can't even read a book about it!
I'd like to write some sarcastic reason for this, but I can't even think of one. I have a feeling that "one nation under God" has something to do with it, though.