The default Adobe DRM implementation requires that you need an Adobe ID (i.e. an account with Adobe) to access the book. (DRM licences are against Adobe accounts)
The bigger players who use Adobe DRM (Like B&N) of course don't want this, so there are alternative schemes where you can use a token to represent the user that is not the Adobe ID.
In the B&N case, I believe that token is based on the credit card details.
You also only download the actual encrypted epub after the DRM licence has been applied. (The initial download is a small XML file which your Adobe RMSDK app uses to decide if you have access to the ebook)
ergo, no valid credit card, no valid account to unlock DRM, and get access to the actual content file.
Ah, the credit card number is part of the encryption/decryption key [1]. Since it seems unlikely that the Nook decrypts the book once and stores a decrypted copy, the Nook also must store the credit card number--or hopefully a hashed version of it. Apparently so [2].
The credit card number is the decryption key. The DeDRM tools for Calibre only require the credit card you used to purchase the book. In contrast, Amazon encryption only requires the serial number for the Kindle you are pulling the books off of.
The bigger players who use Adobe DRM (Like B&N) of course don't want this, so there are alternative schemes where you can use a token to represent the user that is not the Adobe ID.
In the B&N case, I believe that token is based on the credit card details.
You also only download the actual encrypted epub after the DRM licence has been applied. (The initial download is a small XML file which your Adobe RMSDK app uses to decide if you have access to the ebook)
ergo, no valid credit card, no valid account to unlock DRM, and get access to the actual content file.