from what i've read, it's not like: go to vendor, pay gold, get wow time.
rather they will sell an item on the website that people can pay real money for, this item will confer game time, and whoever bought this item can then go sell it in trade of the AH. it allows people who don't have much time to play the game a means of basically buying gold in game while helping people who do have a lot of gold already pay for their game time.
so example... i go to the store on the website and pay $15 for item X. if you use item X, it adds a month of game time to your account. but i don't want game time... i don't play much and don't have much gold, so that's what i want. i go into the game and sell my item X to some guy that wants game time for tens of thousands of gold. now i have basically paid blizzard for the right to buy gold in game and the other guy gets a month of game time.
it's a less offensive way to allow people to buy gold more than it is a way for people to get game time.