This is just an un-researched hunch, but I believe that for the small average sums transacted at McDonalds, the merchant (or the bank) takes on the fraud risk.

This is just an un-researched hunch, but I believe that for the small average sums transacted at McDonalds, the merchant (or the bank) takes on the fraud risk. Thus, they instantly approve your transaction, as long as the number of fraudulent transactions never overshadow the convenience of instant payment and short queues. That's why they don't ask for a signature under $20 in some places.