Free market doesn't mean freedom from regulation, it means freedom to compete. When lack of regulation enables a (set of) actors to make competition harder, you

Free market doesn't mean freedom from regulation, it means freedom to compete. When lack of regulation enables a (set of) actors to make competition harder, you can make a market "freeer" by introducing regulation. Making it explicit to customers with debit cards that they're subsidising the rewards of other customers will [edit: "could"!] incentivise them to demand lower fees, for example. This is a free market at its best.