They don't. The system stores the card swipe as part of a batch which gets uploaded when the aircraft is on the ground. (This is true for the 'cashless cabin' handhelds as well.) Sometimes the batches aren't uploaded until several days later, which is why charges don't always show up immediately.

This happens on the ground as well, although ground-based units (like the kind you'd find at a small business) will call out for an authorization number before the clerk completes the sale at the POS terminal. At the end of the day, the manager would run a batch report and upload all the transactions to the processor.