The credit card processor has terminated merchant agreements with hate groups following this weekend's violence, it said in a statement. (fortune.com)
Terry's right - it's in the Visa / MC merchant agreement that they must accept your credit up to the maximum available on your card. (freemoneyfinance.com)
I believe many merchant agreements prohibit stores from charging different prices for cash or credit, which is why you don't often see that. (money.stackexchange.com)