Re: merchant cannot discount payments made in cash
Actually those laws, and the restrictions, changed years ago. As of January 27, 2013 US businesses were allowed to charge up to 4% more for credit card transactions than cash, provided that they have clear signage about the policy.
Many businesses in highly competitive, low margin trades such as Gas Stations (Petrol for the rest of you on the other side of the pond) have separate prices for cash and credit.
When you only have a 5% margin, 3% in card fees amounts to 60% of the profit!