Well I suppose there are various ways to look at it.
For example fan-fiction writers are already spending time creating these stories and publishing them online for no money, so if they become able to actually generate an income doing something they've previously been doing for nothing then I can certainly see the appeal.
Of course Amazon and whoever created the franchise being written about will gain full control over whatever fan-fiction they publish via Kindle - I'm almost certain that by signing up you would relinquish your own claim to the characters, ideas or scenarios featured in the story.
But even so, is that any different to how it stands now?
For example if the writer of a new Star Trek series read my own Majestic on TrekBBS or Ad Astra, surely they could use it verbatim if they wished with no recourse from me?