It sounds like they are trying to be "Google Books" where you mostly read books online, but Google at least offers the option to download for off line reading on Android.
Barnes and Noble is currently in the process of totally revising their website for purchasing content and also the way Nook books are presented. In early 2015 it will be formally unveiled and likely this change to downloading content is likely a precursor to reading everything exclusively online.
http://goodereader.com/blog/e-book-n...ad-nook-ebooks