Really? Interesting, I never knew that. I know a convenience store and a coffee house that impose a $3/$5 minimum purchase to use a card, respectively.
It's a violation of the merchant agreement for VISA and MasterCard, though I'm not sure about Discover and American Express. (I do know that American Express charges absurdly high per-transaction fees, though, which is why it's less accepted than other cards.)
In any event, while it
is a violation to impose a minimum purchase limit (or, in the case of some small stores in my area, imposing a $1 or so "transaction fee"), it's not exactly hard to get away with such a thing. A customer would need to actually file a formal complaint with the issuer, which would then necessitate an investigation of the merchant (during which time it's rather easy to take down any signage and start accepting low-amount card purchases until the storm clears).
The card fees
do suck for a merchant, though, when dealing with high amounts. My company just recently signed the local electric utility for a Web site sponsorship to the tune of $15,000 ... and the utility wanted to pay with its credit card. Our per-transaction rate, because we process so few credit cards (about 97 percent of our advertising revenue comes in through checks), is 11 percent.
11 percent. Of 15 grand.
(We were able to talk to our bank and whittle them down to a 6 percent fee for this one particular transaction, but, still. Guh.)