This is not true either. All modern POS systems have an offline mode. Have you ever tried to use your card and the employee says 'the machine is really slow t

This is not true either. All modern POS systems have an offline mode. Have you ever tried to use your card and the employee says 'the machine is really slow today'? That's because the machine was offline, but your purchase still worked. In offline mode the POS will still authorize the purchase up to a certain amount, typically $75 (up to the store). Those charges are pushed through after the machine goes online. Yes, it's a failover, but the point is even the modern online system is still not atomic and has eventual consistency. Legitimate purchases outnumber illegitimate purchases by a large factor. It makes sense to err on the side of legitimate purchases. If it were atomic you wouldn't have been able to make that offline purchase, even though you had the money. Since it's true in the majority of cases they let those go through, up to a certain risk factor (the $75 limit).