Despite what Crithto said here

Despite what Crithto said here, it sounds like a bug to me, or at the very least something that was overlooked.
www.euwowgold.com
Hearthstone is a free game ($$ spending being entirely optional), the cross-game promotion for the mount also clearly states that once you acquire 3 wins, the mount is to be made available to you regardless of which version of WoW you play.

That being said, I do have MoP, and an active subscription. So, personally, it isn't an issue for me... I just like to see promises honored.