The Wall Street Journal has a news article that begins, "Merchants may soon begin to impose a surcharge each time a customer pays with a credit card, a practice Visa Inc. and MasterCard Inc. currently prohibit...Visa and MasterCard, which operate the world's largest card-payments networks, ban the practice in the U.S. as part of rules they require retailers to follow to accept their cards."
The Journal doesn't mention it, but these "rules" are either widely ignored or are subject to a lot of exceptions, so far as I can tell. Among the places that charge more to pay with a credit card are private schools ("SMART Tuition also accepts credit card payment with MasterCard, American Express and Discover. There is a convenience fee of 2.5% for any credit card option"), the city of New York's parking-ticket payment operation ("There is a $2.00 per ticket service fee when paying with a debit or credit card), and a lot of gas stations.