In my opinion it's just fine that Blizzard added cosmetic items bought through the Blizzard store, because the items don't give any boost to anything within the game. Your DPS will not get higher, you will not gain more experience, you will not get extra % of loot or anything, it's only looks. There are tons of MMO's around where you actually CAN buy items with real money which improve your in-game performance regarding DPS and such. Look at Age of Empires Online for example.
Also, I can imagine Blizzard is trying to harvest more money in other ways, as their amount of subscribers are lowered compared to last years. What if Blizzard did something else to get their wages from their Subscribers, like raising the subscription costs? I'm sure most of the players would prefer the cosmetic-shop tactic instead of it. Blizzard could also just force you to pay more for your subscription, but they aren't. (Again, there are also lots of MMO's which do raise the subscription costs and force their players to pay more.) You're free to buy or not to buy the cosmetic items and it doesn't change your in-game performance. So in the end, be happy that Blizzard added those free-to-buy cosmetic items instead of forcing you to pay more by raising your subscription-costs. Blizzard had to do something to keep their incomes safe, and I think they made a good decision using this tactic.