"that's a smith and wesson, and you've had your six"
Having worked in a bookstore for a while, this is a prank that shoppers often play. I've seen the Bible, the Koran, Mein Kampf, and various biographies/memoirs filed in sections that someone thought was funny. I'd put money on it having been a shopper trying to make a statement or be funny, with a very slim chance it was the store owner/manager/staff.
I always take a copy of Twilight from the Science Fiction section and stick it into the teen romance section.
I'm hoping at some point it'll stick.