I'm afraid you're mistaken
As I mentioned on Goodreads 38% of people who read and rated it gave it the maximum 5 stars. That's not marketing.
It actually is, though, isn't it? Those positive reviews are a form of marketing just as much as one of those big, attention grabbing displays in the chain bookstores. Word of mouth is a pretty major sort of marketing. This series of books in particular is being held up as an example of "viral" marketing in its most successful form. Like it or not, publicity and marketing are by far
the more important part of being a successful writer. That's true for an artist of any kind, really!
You are right, though; people wouldn't be raving about the books if they hadn't found something they enjoy. More power to them in that case. I truly believe that it can only be because they've never read better, or just don't read very often at all. I'm not passing judgement here (not much, anyway, I promise), and I honestly hope that the popularity of 50 Shades leads to more people reading more often, and a wider acceptance of, and market for, novel-length erotica.
I still think, from what I've seen (plot synopses only, so take it fwiw), that the books are pretty horribly sexist, unhealthy, and outright offensive to me, as someone who identifies deeply with the themes James played around with in her story. But I guess that's par for the course, and I have a pretty thick skin, so whatever. I'll get over it.