Kudos apparents fails to grasp the concept of cross marketing products. Allow me to explain!
Blizzard has Hearthstone, a f2p game with cash options for buying cards. Blizzard also has a MMO with millions of players. Many of those MMO players maybe don't care about Hearthstone, but they like awesome looking mounts, so they decide to play. Blizzard is hoping some of those people will end up liking Hearthstone and continue to play it, thus creating more potential customers.
It's not a difficult concept to grasp.