Barnes & Noble is (still) primarily a paper bookseller -- although they are *trying* to replace that with toys.
People have always been able to browse B&M bookstores and read the books there without paying for them, so the Nook is designed to imitate that capability in order to appeal to longtime paper book readers who are visiting a B&N with their Nook device.
I guess that isn't limited to books with a paper edition.