I did my Uni placement year at Jagex, the company who created RuneScape. Around a year ago they implemented some pretty serious changes to combat Real World Trading, which is against the rules of the game.
Not only for that reason, but there was a whole issue surrounding the gold seller's methods of obtaining virtual money.
It wasn't only gold they sold, but popular items as well, including some members-only items. As the member's game contains members-only items - how else were they able to obtain such items en masse? Pay for membership costs for every bot they needed to send in? Certainly not! Basically - certain gold sellers who were shall we say, less then reputable, would use the credit cards of their customers to purchase RuneScape membership. They would send their hoard of bots onto the members servers to farm these items.
So when Mr. Joe Average gets his credit card bill - he contacts Jagex to say "wtf?" who then refund him his cash, plus a charge to whatever card company they were using. As this was on a huge scale, the costs to Jagex were pretty massive.
Certain organisations would also set up fake RuneScape websites, offering the chance for them to become a moderator, test out the "new beta version" of the "next" RuneScape. All they had to do was enter their pass, etc on the site to gain access. This of course was a huge scam, with the victim's items being traded to gold sellers and the account left with nothing.
Theres also a pretty seedy underworld of child labour/human rights issues involved with the people who farm gold. So basically, anyone buying gold for RS is pretty much funding crime. Sounds a bit extreme, but those were the situations surrounding it.
It's for the reasons above that
this happened to Blizzard.