If it is a public domain book, or the publisher has chosen not to charge for it (e.g. for promo purposes), it ought to be available in a format that your reader can use.
If it's a new book that B&N offers for free for promotional purposes, and it's not made freely available through another vendor, then yes it is illegal to break the DRM and put it onto another device. In these situations, simply because someone (either the publisher, author or store) offers it for free, that does not mean they have surrendered all copyrights. E.g. you can't redistribute it, you can't resell it, you can't make a movie out of it... and you can't (legally) break the DRM either.