I'm not sure I understand why this is a big deal all of the sudden. Don't retailers already account for these costs when they're choosing prices?
The average profit margin for a mainstream grocer (like Kroger) is 1-2 percent. It's a low-margin, high-volume business. Said differently, it's exactly the type of business that I'd expect to care deeply about transaction fees.
They do want to decrease any costs they have, though. The credit cards charging a percentage of revenue instead of retailers' profits or card companies' transaction costs + reasonable profit is ridiculous. Companies like Walmart and Kroger would love to cut that difference out. Given Kroger's growth and low-profit strategy, my initial guess is that blocking Visa would probably be a dumb move on their part. We'll see.