You can still buy Legacy from Forsooth, or you can buy it from AmazonEncore as a hardcover. All of which suggest to me that Amazon has decided to harvest the cream of the self-published crop, and in doing so has made itself into a publisher. The difference is that a traditional publisher relies on editors to determine what to publish, and Amazon is relying on customer reviews and sales figures. Since reviews can be manipulated, I suspect it's looking more at sales, or at reviews that occur at high enough numbers to make them less suspect.
On a further semantic note, Cayla Kluver is a YA author in every meaning of the term, as she herself is a very young adult; she was sixteen when she wrote the book, and she's eighteen now.