Fictionwise used to be a better deal, but they never recovered from the Agency Pricing mess. However, if you look at the multiformat section, you can still find good deals on books that aren't available elsewhere. Many of the multiformat books are smaller e-publishers, but nowadays, more and more of the titles come from companies like e-reads (which publishes e-book editions of out-of-print books) or from smallish print publishers like Circlet Press, Wildside Press, Belgrave House, Dragon Moon Press, and Prime Books. Also, while B&N now has some of the SF print magazines, as far as I know, only Fictionwise has issues of Interzone and the Magazine of Science Fiction and Fantasy (many readers think it's better than Analog and Isaac Asimov's Science Fiction magazine).
As far as the Samhain thing goes, I don't know. I wouldn't be surprised if Samhain signed one of those deals where their prices can't be lower than they are on Amazon or something like that. Thus they have to require that their books be "opted out" of any special discounts.
BTW if you like Marion Zimmer Bradley (as well as Mercedes Lackey and other similar authors), FictionWise has a lot of e-books recently issued by the Marion Zimmer Bradley Literary Trust, including Sword and Sorceress anthologies:
https://store.fictionwise.com/ebooks/p691/Marion-Zimmer-Bradley-Literary-Works-Trust/?
But there are lots of great authors available through the Multiformat Ebooks section.
