That's not strange, that's something Nutaku has no control over. If a bank says to a vendor (-particularly- an overseas vendor selling through the internet), "We believe this transaction was fraudulently made and as such will not be paying this money to you", the vendor often doesn't have much recourse, especially when the disputed amount is for less than $100; it takes time and money to legally argue and the vendor isn't about to shell out more money than they would get. So the vendor, to protect themselves, says, "Okay, fine, but we're closing this account since it's easier than going in and undoing the goods the customer received and may have already benefited from."
Nutaku is far from the only company that operates like this. I don't know if any of you have ever played World of Warcraft, but Blizzard will do the exact same thing if a CC charge gets bounced back as fraudulent. The one difference though is that Blizzard will usually allow the account holder to appeal and explain themselves and if need be, make good on the disputed charge to get the account reinstated.