Well, part of it is the hype and people checking out what's so delicious about the books, but the idea or the popularity of the theme itself isn't new. Half the vampire stuff out there is essentially "weirdly sexual" in nature (Anne Rice wrote several BDSM-inspired novels under a pseudonym), and classics like Lady Chatterley played a substantial role in the "sexual revolution".
I haven't read the Shades books, so I don't know if there is more to their popularity than the lure of the inappropriate. It would surprise me if so many people had a genuine interest in BDSM, so it's probably just a form of voyeurism. (And a novel is less "dirty" than a movie?)
Of course, I may just be totally naive and boring.
