I just redeemed an iTunes gift card code and then tried to buy a movie. But iTunes keeps asking me to enter the details of a valid credit card. I had been buying apps, music and movies on multiple Apple devices without any credit card information. I just used the iTunes gift cards. But suddenly iTunes just would't allow me to do that any more. I wonder why... How can I keep using the money I have redeemed from iTunes gift cards without entering any credit card information?
MacBook Air, OS X Mountain Lion (10.8.2), Core i7 2GHz, 8GB RAM, 512GB Flash