having a pos in places without a reliable internet connection is enough of a reason for stored value cards to be a thing. Some things shouldn't require the mothership to be alive and reachable to work.
You don't need the system to be invulnerable to fraud, you just need to be able to detect it. Offline stored value cards plus separately shipping transactions to a central system with eventual consistency can give you that. The vending machine in question probably isn't invulnerable to physical break in either.
