To avoid this, do not store your credit card information on your iPad, iPhone, or other smartphone or tablet. Never, never purchase anything. Your financial information will be saved, and these apps are highly suspectible to hacking.
If you absolutely have to purchase something in a free app or purchase an app for that matter, use a very low balance credit card. I have a $200 maximum balance card that I got a long, long time ago. I keep it because I use it at sketchy places, like my favorite gyros restaurant and an international grocer. I also use this card for online shopping. IMPORTANT NOTE: Pay off the card in full each month.
How are you keeping free apps free?