Earlier quoted context omitted.
You have to differentiate common purchases from "large" purchases in this discussion. I'm certain you don't think there's something shady going on, or tax evasion happening, if someone uses cash in the grocery store or at Pressbyrån, or to pay for some little gadget or whatever at Kjell&Co. This is what people think of when someone "uses cash". Not hauling tens of thousands to buy a used car or to settle the bill for…
I think they're talking about when merchants insist that you give them cash ("cash only"), not when buyers insist on giving cash. The usual assumption is that if e.g. a street-food cart is cash-only, it's not because they can't accept cards (it's rather trivial nowadays) but rather that 1. they don't want to pay the interchange fees, and more importantly, 2. they want to be able to cook their books when reporting rev…
That seems totally fair if they can’t pass that cost on to you for using a card. Why should they have to pay to accept your patronage?