The new ID system for Nintendo Wii U, Nintendo ID, requires that minors have a valid credit card on hand for identification. According to this news story and screengrab from Joystiq, the credit card is charged a "one-time $0.50 fee via credit card to provide consent for minors to access Nintendo Network." We assume the charge is to verify the credit card as well.
The follow-up confirmation email also tells Wii U Owners that a "Parental Control PIN" can be used to create additional Nintendo Network IDs without any added charges. The credit card information used for the transaction doesn't appear to be stored on the system.
Editor's note: we removed adjusted the story and title slightly to reflect that minors require a credit card to sign up for a Nintendo Network ID.