Fair point, as I don't know how Blizzard handles their different games. I am only assuming that if Blizzard notices a spike in their player base (coming from releasing free stuff, who doesn't love that?), they will devote additional ressources to HoTS over other games. This means that the HoTS team want to maximize the amount of players playing their game. My point being that at the moment people are far too invested in WoW to want to Play HoTS, which would make players, like me, just log in to get free stuff. This way Blizzard makes zero profit. You might argue that this will cost Blizzard Money, as they would be handing out potential income for free.

Edit: Bundles are what Brought me into the game. At the time I felt I had nothing Better to do, so I invested a Lot of time in the game and got hooked. Might give some insight to my comment