Or if the publisher cares about marketability more than quality ....
There's a metric for the former: you can count how many copies are currently flying off the shelves, both real and virtual. There's no metric for 'quality', which reasonable people will always disagree about. Publishers will always care about sales more than the chimera of 'quality'.
I haven't actually read the book, but several of my close friends have, including my girlfriend, and I will reiterate their take on it: it is that it is poorly-written in many ways, but that it is hot, and compulsive. It's very compelling trash - a guilty pleasure - and I completely understand and support both their enjoyment of it, and the decision to publish it.
(I know there are some sticky moral issues around the fanfic-ness of it; I'm choosing to believe, until I'm proven wrong, that it's been sanctioned by Stephenie Meyer in some way.)