Faking the variation shouldn't be hard at all, but how many people would be willing to take the time and risk being prosecuted for fraud in exchange for some free coffee?

Even if 10% of customers abuse the system, it's still a net win for the business. They'll have to cough up some free coffees or sandwiches, but the fraudster is still in the shop and likely to pay for incidentals that go with the promotion; e.g., a drink or chips to go with their sandwich, croissant with the coffee, etc.