I've read before[0] that it is actually a violation of a stores terms with Visa to enforce minimum transaction sizes. Those stores could potentially be reported to Visa and have sanctions placed on them (like an increased transaction rate). Interestingly, a merchant is allowed to offer a discount to people who pay by cash but they can't charge more for people paying by credit card.

[0] - http://www.msnbc.msn.com/id/11697094/ns/business-consumer_ne...