It takes a pretty strong magnetic field to damage credit and debit card magnetic strips without physical contact. If the magnetic field generated by a MMW scanner isn't strong enough to make nearby metal objects stick to it, then it's most definitely not strong enough to damage the mag strip on a card, not even with repeated exposure.

The most common source of magnetic strip failure is not magnetic fields, or even static electricity (though static is a genuine danger), it's physical damage, i.e. scratching, caused by sliding the card in and out of wallets and through card readers on a daily basis.