Blizzard kills over 1000 WoW accounts

If you're a gold farmer in World of Warcraft...
Well, you may not be for much longer. The farmers, who are making gold in the game, then selling it for real currency,
have had their accounts permanently suspended. Over a thousand of them so far.

So, you pay for the game, pay your monthly subscription, only for Blizzard to say "you did something we don't want you to do, goodbye", and they kill your account. That's $50 wasted since, as was reported a while back, you can't transfer the serial number. Once it's used, it's used. Where should the line be drawn on what you can and can't do in a game you pay for initially, and continue to pay for every month? If people are stupid enough to buy a game's currency for hard cash, why shouldn't that be a legitimate activity? Rather than ban the accounts, why do Blizzard not change the game mechanic?