Not true. US law not only allows merchants to provide a cash discount, they may now charge a surcharge specifically for using a payment card.
http://www.jstor.org/discover/10.2307/3203344?sid=2110533961...
http://www.washingtonpost.com/business/capitalbusiness/as-ru...
http://www.nytimes.com/2012/07/17/business/merchants-conside...
http://usa.visa.com/personal/get-help/checkout-fees.jsp