IIRC Visa (at least) has a rule stating that a business can’t charge more for Visa card transactions than cash transactions, nor can they have a minimum purchase for Visa card transactions (unless they also have the same minimum purchase for cash transactions.) If they find out a business is doing these things, they’ll stop allowing the business to accept Visa (even through intermediaries like payment processors).
I've reported a few businesses and it was an easy process. I don't think it's an enforcement issue; more of consumers not being informed that the merchants shouldn't be forcing them to increase their basket size to a min amount and not knowing that it's easy to report.
