Why do I have to give credit card information to download a digital copy of a movie?

Is it an account that you've just created ? If it is, then unless the instructions on this page are followed when creating an account : http://support.apple.com/kb/HT2534

then credit card details will need to be entered before the account can be used to download any item from the store.

If you are being prompted to review your account then you could see if this post by mountaingoatgirl lets you do so without needing to enter credit card details : https://discussions.apple.com/thread/5295285?answerId=24303054022#24303054022