Yeah, you can argue it's not your Library, it's Amazon's library. Cause technically, you don't own the book you are using on your kindle, you're just leasing it

Yeah, you can argue it's not your Library, it's Amazon's library. Cause technically, you don't own the book you are using on your kindle, you're just leasing it's content for viewing. If you did put the book there yourself (obtained externally, say a .mobi file), Amazon would not be able to take it away. You own the device hardware, you own whatever you put on it. But it would be unfair to the author if you got to keep something that Amazon "stole" from them, even if you bought it from amazon. But this would be a issue with any content with the current IP laws. And isn't anything wrong with the Kindle itself. Imagine if you bought a book, photocopied it, and returned it. This is what letting you keep the book will be equivalent to.