I've read some rather long discussions about this perplexing (to me) fact on the Amazon boards. The general opinion was that her high sales figure was due to pre-ordering and that it didn't keep up afterward. She fell from the best-selling list pretty rapidly (in comparison with Stephenie Meyer). I don't know how true this is since I couldn't be bothered to check.
Her right track seems to be along the lines "sex sells". And if she's educated an audience who thinks she's edgy and a source of information about sex, then there's no surprise she still sells. But she's lost almost all of her original fanbase that made the AB series famous in the first place. On a personal note, I would weep if this happened to me.

People read and buy bad books: sad, but true fact of life. Goodkind's sales are quite good as well.
PS: I never paid any money for their books. Long live illegal ebooks.