Seeing as it doesn't reproduce the functionality of the Hearthstone client or server in its entirety, I think it falls in the same category as fansites such as Hearthpwn. Blizzard could probably pursue a successful takedown, but I can't see a good reason to put resources there when there are things like WoW private servers that pose a legitimate business threat.

I think it's not a problem... This is not a playable clone of the game. It's purely for deck analysis, kind of like how SimulationCraft is a DPS simulation and analysis tool for World of Warcraft.