This begins to deal with complicated issue in fandom: money. Fans already spend money to enjoy whatever it is they fan over, whether through going to movies, buying music, paying their cable or electric bill, or buying a computer and wifi connection. The fandom itself, however, hopes to get some return for it. Artists sell prints and sketches, as well as putting their drawings on t-shirts, bags, hats, etc. People create things to sell that are related to their fandom. This competes with what merchandise might be authorized for the cultural object and feels a little bit like buying local - you're supporting an independent artist rather than a larger corporation and getting something rather unique. However, this same idea doesn't seem to be expressed when it comes to writing.
Perhaps its because we're so used to getting fanfiction for free on the internet. With free fanfiction sites, it would seem like selling fanfiction wouldn't actually work. Because art can be shown on the internet for free, writing is shared the same way. However, one can buy a print but can download a free copy of a fanfiction story. There's a sort of difference between the drawing and writing in fandom and a difference in how we think about the two things. We're willing to pay for the art, but not for the story. Even as a writer, I'd love to have people pay for my writing, but I don't feel willing to pay for someone else's and find the idea of selling my fanfiction rather odd.
In short, Kindle Worlds is an interesting concept but one that would require a redefinition of how fandoms are currently operating. Will it work? It's too soon to say. But it continues to bother me that this began after Fifty Shades became a hit and that the focus seems to be on buying products related to certain fandoms, rather than supporting authors who write about certain fandom characters. In a way, it seems like a way to extend marketing. In fact, I would be curious to know if only certain fandoms are accepted for publication on Kindle Worlds and whether this is connected to a certain agreement with Amazon and companies that own the rights to specific characters. I'd like to think positive about the whole thing, but as I'm very connected to ideas of fandom that aren't about money and are working to make mass culture more like folk culture, it's a tendentious issue for me and one I'm continue to have mixed feelings about.