I think 3% is an exaggeration. And in fact, you can get 2% cash back. I don't think there's very much being charged when all is said and done.
This seems to show the fees can be considerably less than 3%, although PayPal or Square will charge nearly that:
https://www.fool.com/the-ascent/research/average-credit-card...
https://www.mastercard.us/en-us/business/overview/support/me...
That's an interesting point. I'm not aware of any POS payment processor used by business that accepts debit cards but not credit cards. Not sure why that is.
Debit cards circumvents the need for cash as well and are much cheaper. Everyone uses debit cards where I live, cash is very rare so companies doesn't need to deal with it much.
So the difference is really just a straight 3% vs 0.15% with no other consequences.