B&N employee here to shed some light on this.

That item is in the Bargain section of the store. Bargain, for those who don't know, is where we have our discounted books. They're not 'On Sale' per se, but they're discounted. The way Bargain works is as follows. Publishers print a load of books. More than they'll need most of the time. We sell those books in the store but rarely sell all of them. After a while, the publishers request them back from us because for many titles they intend to print a paperback edition or one with a new cover that ties in with a movie or tv show release or whatever. In that case they'll want the old stuff off of the shelf and the new stuff on the shelf. However, there's only so much space in warehouses and distribution centers to store books and the publishers can't be bothered to store hundreds of thousands to millions of books that are 'old'. So what happens is we sell them for a very discounted rate in our Bargain section. This helps push inventory of older books. There are some titles that are printed specifically for the Bargain section but I won't talk about those, unless for some reason somebody wants to know.

Generally speaking it's a good program all around. The only catch is that stuff doesn't necessarily stay in Bargain forever. And books from the initial print run may wind up in Bargain whereas the same book from a later print run might be sold for its MSRP later on, resulting in the same book being in two places in the store with two different prices. Bargain titles get their own ISBN (unique identifier for any given book) that's different from their original so that we can track them differently in our inventory system.

So for this title, it's likely that Blizzard had a lot more of them printed than was necessary, didn't sell all of them in their initial release, and their publisher/distributor is in league with B&N to sell them for less than their MSRP so that they'll cease to be dead weight in a warehouse. There are quite of few copies of this title in warehouses still (I looked it up recently) so if you're interested in one you shouldn't have an issue getting one from your local B&N or from bn.com at the $20 price point.

Hope that is somewhat helpful.