I don't know now--but when credit cards were first being offered in the US--the card companies got the US Government to write a law that prevented any stores from charging more for credit card transactions than cash or check transactions (even as a kid--it seemed like the retailer got scr*wed because it was obvious that the CC company was making ~5% on the deal and the vendor was not able to collect for it)...
However, even to this day, it is legal to offer a discount for cash (don't know if the merchant agreement is enforceable--but who would want to pay a lawyer to fight it). Gas Stations still offer credit/cash pricing and, for large purchases (years ago), I got them to knock 5% off for check.
-Bill