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

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.