Credit card customers spend more on average than cash customers, and yet the instant merchants were allowed to pass on that surcharge, they did. Larger merchants will likely continue to absorb it, yet those merchants probably have preferential pricing anyway. You absolutely underestimate the short sightedness of Australian and NZ merchants.
It is unlikely that they would be permitted to do this if they were regulated. They'd be forced to treat everyone equally, but there'd almost certainly be a "surcharge free" filter ;)
Afterpay actually have said in their response to the RBA that the majority of their customers have a debit card attached - not a credit card. This mans the interest free period on credit cards isn't relevant, and also that Afterpay isn't paying as much to process the card.