Apple has begun sending emails when Apple ID’s are used to make purchases on iOS devices not previously associated with the account. It’s “likely these emails are being used as one way to combat increasingly frequent app purchase fraud,” says “MacRumors” (http://www.macrumors.com).
Apple already requires credit card users to reenter the three or four digit CID number from the back of their credit card (front in the case of American Express) to authenticate the new device. However users with balances from iTunes Store Gift Cards aren’t required to perform any special authentication other than entering their AppleID password.