For adults without kids this may sound a little silly, but in the past year I have made several purchases based on this simple fact. My kids use Kindles because the older ones can have an Amazon account without attaching a credit card, when they need money I can quickly and easily email them a gift card. Barnes and Noble requires a credit card be attached to the account for your eBook reader. My kids use iPod Touches and iPhones, much to their dismay, (They had both picked out an Android device) because they can use gift cards with iTunes and Google has no such thing for the Android Market.
It comes down to the simple fact that Apple (through a little trickery) and Amazon do not require you to have a credit card on file with your account. To go along with this each make getting a gift card relatively easy, you can email one from either service and for iTunes you can get a card about anywhere. Thus, protecting my credit cards from being used to buy "Smurf Berries".