Q: OS X Mavericks is offered as a free download, why is apple asking for a credit card number?

I was thinking there might be an outstanding balance. That is, an in-app purchase had been made after the credit card expired, and the App Store was waiting for an opportunity to bill it. But since there is no balance, there isn't anything else you can look into yourself. Time to contact Apple Customer Service. Click on the Purchases, Billing & Redemption button, then decide how you want to contact them.