Starting this week, Blizzard is rolling out an account-wide change for everyone who has parental locks in place -- add-on purchases (read: microtransactions) will be disabled by default. This is most notably for Hearthstone: Heroes of Warcraft, which is built on microtransactions, but also prevents Diablo III Auction House payments until it closes down in March, and World of Warcraft purchases.
This is pretty much a standard practice these days, even in the mobile market. The recently released Angry Birds Go! even has warning logos for add-on DLC purchases in its loading screens. Madness!
Battle.net Parental Control Changes [Battle.net]