I was going to link to The Good Guys and their "Pay Cash Pay Less" marketing, but it appears they scrapped their cash discount after finding customers increasingly prefer to use electronic payments:
http://www.smh.com.au/business/the-good-guys-scraps-cash-dis...
That's a weird reason. Surely that's the whole point of the discount? If customers preferred paying cash, then no discount would be required.
I think the point of removing the cash discount is they can use the 'extra' 3% margin earned on those remaining cash sales to subsidize lowering their price via credit card to be more competitive. Their business is based on offering the lowest prices, but the cheapest price was cash only. The discount for cash no longer lures customers to come to their stores, so there's no incentive for the store to continue passing on the discount to the customer.