There are businesses dedicated to buying, selling and producing it. It can also be exchanged for goods and services, is easily valued in and converted to USD and other currencies, and fluctuates in value depending on supply and demand.

I picked WoW instead of a game like EVE Online because scamming is not an expected part of the game.

I'm not aware of any criminal proceedings against people who stole WoW gold, but it is an entirely different matter if actual money (i.e. USD) was taken.

Sure. Like I said, maybe it's a crime, but not one hugely worth prosecuting.