Do you really think that banks and card networks work for free?
Even if you use the best rewards card and never pay a cent of interest or fees to your bank, you can bet that the program on aggregate is running a profit for the issuer, or they would not be offering it in the first place.
In effect, your rewards have to be either paid by the merchant (i.e. you, in the end, in the form of higher prices for goods and services), or by other cardholders in the form of interest, fees, using a suboptimal card/rewards category combination etc.
> Do you really think that banks and card networks work for free?
They certainly don't work for free, I addressed that above.
The credit card fees the merchant has to pay are bundled in the price of each item, so you can't escape them. By using a cash back card you can recover some of it, so you're leaving money on the table if you pay by any mechanism other than a cash back card.
A few gas stations around here offer a discounted cash price. In those, I'll pay cash.
You're not paying anything for the protections the credit card offers.
Or more precisely, you are but we all are, even if you pay cash or with a debit card, since the vendor prices the items accordingly and there's no cash discount. (There are exceptions, but very rare in the US.)
And most credit cards give you cash back so it's actually cheaper.