It has to be decentralized because you don't have to connect your kindle to amazon at any point in order to purchase and read a book.

You can choose to download the book manually from amazon when purchasing and move the file directly to the kindle over USB and be done.

The book is encrypted with your device serial number (or something similar), which is also why, if you would want to strip the DRM from the file (which might be illegal, depending on where you live), you have to supply your kindle serial number to be able to decrypt it.