Good for the retailer, but generally bad for the customer.It's a bit more complicated than that. Retailers likely do have to pay extra for contactless when a customer uses a debit card simply because they have to run them as credit. (Prompting for PIN with debit cards in Apple Pay makes for a poor customer experience.)
Using your card as a debit card is generally a bad idea. Unless you're getting a discount for swiping as debit, don't do it. Swipe it as a credit.
You get more protection when you swipe as credit.