Just great, not that I care about SC2, but if they'll pull the same joke on Diablo 3 I'll be seriously pissed off. Games used to be like any other type of free time activity: Whether it's a movie, a board game or even playing with a damned ball you could do it with others for the same price, as long as they're sitting next to you.
In my opinion this is simply fair use of a product you bought. I'll want to play it with my GF, and I seriously fail to see why we should pay double, it's not like we had to pay extra for games with a splitscreen function either, right? Oh well, let's see if there's workaround. In worst case for me I'll be picking up 2 copies from a cheap webshop, like G2Play.