I wonder if Amazon can get sued for selling copyrighted material they didn't have the rights to sell. I know about DMCA and "safe harbor" laws... but I'm not sure they cover when it's your store that's doing the money exchange. AFAIK, Amazon doesn't claim to be a "provider of ebook selling services;" it tells customers that *it* has provided these ebooks.
Safe harbor protects the post office from being arrested for delivering criminal materials, but the post office doesn't claim to be providing those materials; it's very clear that they're just the messenger. Amazon expects customers to come to it, not the book creators, for their Kindle books.
Since Amazon can't be certain of getting rid of the distributed books, which may have been copied & converted many times already, they seem to be at least as culpable as Grokster or The Pirate Net--they're providing a service that allows, possibly encourages, people to share copyrighted content.