As you have discovered, free does not necessarily mean DRM-free.
B&N ebooks are ePubs, but with B&N's own password-based DRM scheme that involves info from a credit card (name and number).
Sometimes an ebook is free in several ebook stores, so you might be able to find it with Adobe DRM.
My advise is to always strip the DRM (Adobe or B&N) before importing to Calibre. That way you are safe if Adobe or B&N gets out of the ebook business (Adobe abandoned DRMed ebooks once before). This can now be done completely transparently using 3rd party plugins to Calibre. See Apprentice Alf's Blog and the tools_3.0 bundle. Note that this also works with Kindle ebooks via K4PC.