Perhaps you should take up the issue with your credit card provider, and post their explanation back to the forum. Should or would be very interesting?.
Personally I would suspect that this is to confirm and provide written evidence that the actual sale was provided at sales point. Your (customer) copy would be disguised to cover protection through loss of voucher?.
Handing over a credit or debit card at restaurants and filling stations always have an element of risk (or at least that's often suggested especially if it vanishes out of your sight for awhile)?.