I don't like to defend Amazon
But Amazon hosts the ebooks on their servers and sends the ebook to your device, as well as sync everything. I have checked out a book via Overdrive and Kindle/Amazon and months later checked out the same book as I didn't finish it, and Amazon knows where I left off in the book, which is cool. Yes, Amazon hopes you buy the ebook from them, and by 'managing' it they get you to their web site, so they have some advantage, but also some responsibility managing it all. I think it is a fair trade at this point.