I just saw this as a loss leader (business/marketing term). It's when you offer something really good to get people to pay attention to you, and the business actually eats the loss of the sale, in hopes of having a long term customer they can make money off of.
Famous Loss Leaders are anything advertised for Black Friday that's a super great deal, that they only have 20 of, so people wait in line. The idea is, if you've invested that much time in line, even if they run out of the item you want (that HUGE screen TV for $10), you're still going to buy something else from the store and they make money off of that.
Still debating about GW2... I do'nt really have the time to devote to leveling up quickly a character for end game fun, but this game seems to have more of a story than just trying to get to the max level...
EDIT: Greenmangaming is out of GW2 for purchase at the moment
I'd guess they won't have any more in that would qualify for the 20% off, but who knows...