The biggest problem I can see with Blizzard selling gold is two-fold:

1. It would greatly reduce incentive for players to run dailies/randoms for gold when they could skip the work by paying real cash. They have enough troubles making people like this "casual" content enough to do it that they likely don't want to introduce another reason not to.

2. Depending on how many people buy in, it could lead to such severe inflation that commodity items like blue-quality gems soaring in price to thousands of gold, making it impossible to enhance your gear without massive effort, or massive cash. And Blizzard just cannot anticipate how many people would buy gold if they could.

If they wanted to go with some gold-buying scheme, the only reasonable way to do it would be to allow players to sell to each other like the RMAH in Diablo 3 will allow. It's not a popular idea, and it would give the illegal gold-sellers a legitimate way to sell stolen/bot-farmed gold for cash, but it does avoid most problems with inflation while allowing players more direct control over how much gold they have.