Maybe smaller places that can't absorb the merchant fees as easily, but big retail places don't usually charge more for using certain credit cards. It'd probably cost them a chunk of business if they did.
I think what the parent commenter was trying to say is that merchants raise prices on everything, whether you pay with card or cash, to offset credit card fees. What ends up happening is that customers that pay cash are paying for the rewards I get by using a credit card.
Their prices will be set to absorb the average mix of rewards and non-rewards card that retailer sees. They're certainly not going to consider it a charitable donation to high credit-score folks.
It'd be much fairer if they surcharged rewards cards, or if we went the European approach and capped interchange fees at something reasonable. The current system is basically a regressive tax on people with lower credit scores.
Or their merchant account. I think they disallow charging different prices based on the card type - there was a big blowup a few years back because retailers didn't want to pay the higher fees to accept debit cards, but had to because the debit cards had VISA or MasterCard logos and therefore had to be treated the same way as VISA or MasterCard credit cards.
