you do pay for it in shops. which is where the old “discount for cash” came from
No, no it isn’t at all. The “discount for cash” is about avoiding putting the transaction through the books, usually to avoid taxes, be that of the value added, income, or corporation variety.
The credit card surcharge is just that – additional charges to cover the additional transaction costs of that specific form, this is why you will sometimes find charges for credit, but not debit, cards. Also why you get a lot of retailers not taking AmEx since the transaction charges for that are even higher. It’s a bit annoying when it’s an add-on and credit card is the only way of paying though, but this won’t generally be the case for shops.