Is there any duty of care for the vending machine software vendor towards the owners of the vending machines? Are they not playing the role of a virtual clerk that allows shoplifters to walk out of the store unchallenged whenever they yell out "It's okay; I've already paid for this!"?

It's worth stealing a $1 coffee to expose the extreme negligence behind the virtual clerk software. The software is essentially turning the vending machine into an honor box, and presumably the owner of the machine actually wanted proof of payment before vending anything, or they wouldn't have bought the machine. They could have put up a mains-powered samovar with a coin box bolted to it and a sign reading "1 euro per coffee. Call (+39) 355 5555555 to report problems."

It's not even clear to me who is being stolen from. How does Argenta determine how much they are to pay the machine owner? How do they determine how much to pay the machine servicer? If Argenta pays for the coffee, and the owners and servicers are unharmed, potential theft of coffee becomes an incentive to repair their software. Otherwise, you'd just be screwing some vending machine operator whose only failing was to trust Argenta over a dumb(er) coin and note validator.