Which is PRECISELY why I refuse to buy more (or put back my) stuff when a clerk tells me there's a $5 minimum and I only want $2.50 worth of stuff.
A merchant has many obligations. Lease agreements, supplier terms, etc. Most don't really affect me directly - but I KNOW they are supposed to treat credit cards the same as their 'normal' purchases. And when they don't, they break the promise the cards made to me, and they certainly inconvenience me. I walk into a store that sports a credit card sign, with a valid card in my pocket, and expect equal treatment.
Now - you might say "what about gas stations that offer 5 cents per gallon off if you pay cash" - that's a little different - there, cash is segregated from other forms of payment, whatever is normally accepted, and treated differently. This isn't just semantics - it means that my credit card is still equal to a check, a fleet agreement or whatever other payment terms the gas station accepts. And that is contractually OK by the credit card terms.
When they tell me I can't buy my chips and soda when I'm attempting to buy them with a card they accept - that's a problem. Which makes it a customer service issue, which is why I've not returned to those places. I'm not hung up on it, but I'd suggest to other merchants to not do try this ploy.