"One plausible explanation: Hackers familiar with the technique are selling access to hacked iTunes accounts with store credit to burn. Perhaps if you're willing to pay a hacker $10, he'll give you access to a hacked account with $50 of credit—and perhaps Sega's game proves quite popular with folks willing to make that deal."

I can't imagine they are selling accounts to people who are actually using them and no one outside has noticed. If you want to steal apps there are easier ways than trying to purchase a compromised iTunes account. Stealing an account or purchasing a stolen account is a good way to get in a lot of trouble (it also shouldn't be hard for Apple to figure out who's doing the in-app buying--you have their device ID). This is for in-game credits, so simply jail breaking and installing from Hackulous won't do the trick, but I have doubts that a bunch of people want to risk going to jail just to play KingdomConquest on iOS.

They're converting in-app purchases to cash somehow.