I like mounts. I like to collect mounts. I like to support Blizzard. I have money.
Therefore, I combine these things and buy a mount from the store. It makes perfectly logical sense and I have 0 problem with it. I don't care that they spend a minuscule amount of time making these mounts compared to everything else they do. As someone else said, they've added..what...3 mounts since MoP? That's insignificant. Sure, they could have spent that time adding those 3 mounts in-game, but they'd have to spend more time figuring out how to add them, tieing them into an achiev, or new faction, or new event. They'd have to make logical sense to be earned in game. The store gives the artists the ability to be creative and add mounts to the game they otherwise would or could never add.
/2cents