I don’t know how long this has been in effect, but it was news to me. You can no longer turn public domain books into Kindle books and publish them on Amazon:
Public Domain and Other Non-Exclusive Content
Some types of content, such as public domain content, may be free to use by anyone, or may be licensed for use by more than one party. We will not accept content that is freely available on the web unless you are the copyright owner of that content. For example, if you received your book content from a source that allows you and others to re-distribute it, and the content is freely available on the web, we will not accept it for sale on the Kindle store. We do accept public domain content, however we may choose to not sell a public domain book if its content is undifferentiated or barely differentiated from one or more other books.
This will have the effect of keeping out the junk, but at what cost to the reader? And at what cost to those of you who were making a living turning old books into Kindle books?