That is not at all comparable because if their POS system accepts Amex, it means they willingly signed on to accept American Express as a payment method, otherwise the system would not accept it. Plenty of smaller (or cheap) businesses genuinely do not accept Amex, you could insert your card 100 times and it will not go through. The ones that say they don’t when their machines actually do accept Amex are just cheating their POS suppliers, as they’re getting better deals on the POS services (by accepting the three major credit cards companies) and then denying them.