If I have a card that gives back 2% to me, back causes 5% fees to the vendor, both of us would be better off if I used a card with 1% fee, and the vendor gives me 2% discount. Unfortunately, not allowed.
The real reason that most merchants don’t charge surcharges is that they don’t want to lose the sale, calculating the actual interchange is wildly complex and in general they prefer cards to cash.
It’s a noticeable fee for them.
As far as I know they're also allowed to charge surcharges for non-capped cards (e.g. business/commercial cards, three-party schemes like Amex etc.)
What people forget about these fees is a credit card is cheaper to take for the merchant. The credit card is never counterfeit money. The clerk never takes money from the credit cards, nor does the manager counting it (I wasn't in retail long but I saw both). You never have a robber come in to take your credit card money. Even when all goes well, you don't pay the clerk and manager by the hour to count all the cash twice. You do have some risk of taking a stolen credit card, but overall it is cheaper for the merchant to take credit cards and that savings should be what pays for the card costs (I have no idea how to count the different costs to see if that is true)