So do retailers like paying the fees for other cards, or is the headline just apple bait?
American Express isn't Visa or Mastercard, they are all on different payment network. I have no issue with them having different fees because they are different.
The Apple Card is a Mastercard. Does Mastercard have different fees based on the type of card? I would certainly hate Mastercard more if that was the case.
Visa and Mastercard both do. "Premium" cards with more rewards have higher interchange rates.