So, I looked at the submission rules, my take on each one:
Pornography: We don't accept pornography or offensive depictions of graphic sexual acts.(Ca
Offensive Content: We don’t accept offensive content, including but not limited to racial slurs, excessively graphic or violent material, or excessive use of foul language.(Tackling both these at once, it looks like a good idea on paper to establish these rules, but what about works that the source material contains these things? Especially the later for horror works. Does that mean people can't have things lopped off in a Highlander fic, despite that happening in even the prime-time Tv Show?)
Illegal and Infringing Content: We take violations of laws and proprietary rights very seriously. It is the authors' responsibility to ensure that their content doesn't violate laws or copyright, trademark, privacy, publicity, or other rights.(I really have no idea what this means when the fanfic does that by nature)
Poor Customer Experience: We don't accept books that provide a poor customer experience. Examples include poorly formatted books and books with misleading titles, cover art, or product descriptions. We reserve the right to determine whether content provides a poor customer experience.(Nice rule, I agree)
Excessive Use of Brands: We don’t accept the excessive use of brand names or the inclusion of brand names for paid advertising or promotion.(Define Excessive?)
Crossover: No crossovers from other Worlds are permitted, meaning your work may not include elements of any copyright-protected book, movie, or other property outside of the elements of this World.(So, you're alienating about have of all fanfic, then)?
It also says:
when you submit your story in a World, you are granting Amazon Publishing an exclusive license to the story and all the original elements you include in that story. This means that your story and all the new elements must stay within the applicable World. We will allow Kindle Worlds authors to build on each other's ideas and elements. We will also give the World Licensor a license to use your new elements and incorporate them into other works without further compensation to you.
World Licensors have provided Content Guidelines for each World, and your work must follow these Content Guidelines. We strongly encourage you to read the Content Guidelines before you commit the time and effort to write.
So, yeah, the fine print amounts to "we're going giving you so many limitations it will make your head spin and give you the shaft as far as author's rights" You can't even use your OCs or OC elements outside the platform once you've published a fic with them in it on it.