why do i need to give them my credit card when i just want to redeem a gift card

I have itunes set up on our home computer. We got a iTunes gift card, so we tried to buy an album I entered the verification code on the back of the card. the top righthand side of the screen successfully registered the $15. Then when I went to buy an album, it wouldn't let me unless I entered credit card information. Why does it demand my credit card when I am not paying by credit card? When you buy something at a store with cash or a gift card, they don't demand your credit card.