We have this for the same reason we can't have violent cartoons. People are convinced we're all idiots and can't tell fact from fiction without censorship. Oh, and parenting, they think we can't parent our own kids, so they have to make sure our kids 'grow up right' by telling them what to think rather than teach them how to use reason and logic to deduce the truth. ie, In the fiction section I find Star Wars/Trek novels (that's not real???? no way!), Literary classics (now, I know Sherlock Holmes was a real detective!), and many many many books written about aliens, talking animals, future events, time travel, etc. (all things I see EVERY day). I can use reason to know that a book found here is lumped in with these books because it's not real, or I can go to the nonfiction section and buy a book for $30 to TELL me it's not real (what a bargain!)
Fun Fact, in 2005, New Zealand actually put it to a vote to decide if people could say that being a JEDI was their religion!