I compare the ebook readers as a physical object with the books (also physical objects). If I can read books on that device even when my credit card has expired or other things have disabled my account, then IMHO the parity to the real world exists: I have that one physical object which represents the content on it. If I lose it and don't have my account any more, well, too bad.
Of course, by centralizing many books to one physical object, you risk losing much more content as you lose that physical object. But the advantage is that, in case it breaks, it can be repaired (which might be impossible with a physical book)
Now. I know for a fact that this is how Kindle works: even if your account is gone, your books stay on your Kindle and you can continue to read them. I would assume it works the same for B&N in this case.
Being able to infinitely redownload the books is a nice convenience that goes far beyond what a physical book would allow.
Also, I haven't tried it, but these DRMed books are just files after all. It might be possible to back them up and later read them on the same device (again the physical book analogy) even if the account has been terminated.