Being asked for credit card information for updates on apps already purchased
Whenever you use your Apple account they will try to validate the information in your account. If there is missing address information you will be prompted for it. If you haven't agreed to the latest Terms and Conditions you will be prompted to agree. If you have credit card information on your account Apple will try to validate it. If the information is out of date, you will be prompted to update it even when installing a new free application or upgrading old applications.

You aren't being charged for updates.