Charging a credit card fee to customers using a credit card is not illegal as long as the merchant says that they are gonna charge the credit card customer a surcharge fee to use the credit card.
You always have a choice to go to another store or merchant to use your credit card.
But in reality you're paying a credit card fee anyway through the purchase price of your merchandise that you buy.
All merchants charge a credit card fee to all customers through the price of the stuff they are selling.
So you may not realize it but you already pay the credit card fee when you buy something.
All merchants are charged a percentage of the transaction to process the credit card transaction so they have to pass that onto customers.