"BNPL providers can offer zero-percent interest rates because they charge merchants three to four times the average credit-card processing fee."
Right. And the vendor can't charge a higher price to the BNPL crowd, so they average it out over everyone. And thus the BNPL app user gets an "interest free" microloan, with everyone, even the people paying cash, having to pony up for the interest.
Just like credit cards with their incentive programs... the card companies are skimming 2-3% off the top anyway, might as well capture some of that for myself.