As far as I can tell, Adobes ADEPT DRM is based on publick key cryptography, so when you generate an ID, you get a public key, and a private key. When the bookseller receives your public key (via Digital Editions, the UrlLink.asc file tells DE where to send it), it can then encrpyt the file so that only you with the private key can decrypt it. This public/private key pair would not necessarily rely on any particular email address.
Have a read of wikipedia for a more detailed insight
http://en.wikipedia.org/wiki/Public_Key_Cryptography