Answer: along with many less-known authors, they have all had their sales rankings removed by Amazon.com (and perhaps Amazons elsewhere - I've not checked). The reason, as given to YA author Mark Probst, whose book The Filly received the same treatment, is as follows:
In consideration of our entire customer base, we exclude "adult" material from appearing in some searches and best seller lists. Since these lists are generated using sales ranks, adult materials must also be excluded from that feature.
So, homosexual love is now considered 'adult material' - even when it's in a YA book, and even when it involves no explicit sex at all. Needless to say, you can still find plenty of heterosexual erotica, sex toys, etc on the Amazon catalogue, with rankings displayed as proudly as any throbbing member.
Considering the importance of Amazon sales ranks and best seller lists, this amounts to nothing less than a deliberate attack on GLBT writers and readers. It's not just homosexuality that's got Amazon's goat, by the way: Julia Serano, Helen Boyd, Leslie Feinberg and Kate Bornstein, amongst others, have also had their rankings pulled. You can't accuse Amazon of not covering the whole GLBT spectrum!
Feel free to give Amazon hell. I'll be removing their links from my own web site as soon as I've posted this.