It isn't just that they're buying non-genre YA fiction. It's an indicator of what Twilight really is.

Most "also bought" lists on Amazon show relation by author or genre. If I use the "also bought" list to classify Twilight's genre by eliminating books by the same author and books on the order of Everything I Ever Needed To Know I Learned By Reading Twilight, it's probably YA fiction, rather than fantasy or horror.