Every time Blizzard announces something new for the pet store we see the same threads come up about how a game with a monthly sub fee shouldn't have a microtransaction store. I get really frustrated when I see these posts because some people just don't get it.
You pay a monthly fee to be able to log in to your account and play; nothing more, nothing less.
Blizzard is a business and they have to do with that money whatever they feel will be most profitable. This can be anything from developing PvE and PvP content to developing items for the Blizzard store. The purpose of the former is to keep people subscribed to WoW and maybe even bring old people back/new people in. The latter is designed to bring in additional revenue to be able to develop more content.
The most important thing to realize is that Blizzard will NEVER make Tier Sets or any current content gear available for cash; they are not stupid enough to do that and you can mark my words that it'll never happen.
The Blizzard store is not a slippery slope, it is a smart business move to bring in more money for the company. The more resources Blizzard has, the more content they can develop and the better off we are as players.