A couple things..
When a retailer accepts credit cards, EVERYONE pays for the cost (3%-5% typically) for accepting these cards. It is against the VISA merchant agreement to charge more for customers to use their cards (This takes away the 'penalty' imposed on the customer). So, the retailer raises prices so that overall, the customers who pay with cash subsidize those who pay with credit.
On a side note, you'll notice that if you live in California (im not sure about other states), you can't pay your utility (electric/gas/water) with a credit card because the government won't allow the overhead to be rolled into the rates for all customers, and the companies aren't allowed to directly charge the customers the 'overhead'.
You will notice that you can pay your cable bill with a CC, since they aren't regulated and are free to rip customers off for as much as they can.
It is also against their agreement to have a minimum purchase amount. (Tell the liquor store you're going to report them next time they tell you that you cannot use your credit/debit card to pay for that candy bar instead using your ATM card and being charged the $.45 they want to rip you for).
