Amazon has not yet responded to yesterday's story about a book-stuffing scammer, but they have quietly changed the rules to make it more clear that book stuffing is not allowed.
According to the new rules bonus content can make up no more than 10% of an ebook:
If you choose to include bonus content (e.g., other stories, or previews of other books that are not part of your book’s title), it should be relevant to the customer and should not disrupt the reading experience. To meet these guidelines, we require placing additional content at the end of the book, and listing the bonus content in your book's table of contents.
To provide an optimal customer experience, bonus content should make up no more than around 10% of your book. If you would like to include multiple stories within your book, consider creating a collection of works. When selecting your book’s title, always make sure to follow the Metadata Guidelines.
Primary and bonus content must meet all program guidelines (e.g., bonus content in KDP Select titles must be exclusive). Translated content must be high quality and not machine generated. Disruptive links and promises of gifts or rewards are never allowed.
You can find a previous version of this page in the Wayback Machine.
The rules change was inspired by yesterday's story about a scammer who is cheating Kindle Unlimited via the book-stuffing con. They were stuffing whole novels in the back of each book, effectively making the book an anthology mis-labeled as a single book, and then collected the maximum payout in Kindle Unlimited by tricking readers into visiting the past page of the ebook.
Amazon has not yet commented on yesterday's story, nor have they banned the scammer. It's not clear why; the scammer is clearly violating both the spirit and the letter of several existing rules.