There are still laws at the state level in the US that make it illegal for a merchant to charge more for credit cards. I lived in Colorado when the law passed there. It was impressive marketing by the credit card companies under the guise of consumer protection. Sucks because most people fell for the propoganda and now it's law.

  Colo. Rev. Stat. §5-2-212 
  “(1) Except as otherwise provided in §§24-19.5-103 
  (3) and 29-11.5-103 (3), C.R.S., no seller or lessor 
  in any sales or lease transaction or any company 
  issuing credit or charge cards may impose a surcharge 
  on a holder who elects to use a credit or charge card 
  in lieu of payment by cash, check, or similar means. A 
  surcharge is any additional amount imposed at the time 
  of the sales or lease transaction by the merchant, 
  seller, or lessor that increases the charge to the buyer
  or lessee for the privilege of using a credit or charge
  card. For purposes of this section, charge card includes
  those cards pursuant to which unpaid balances are 
  payable on demand.”