That comes out of the merchant's pocket, who has priced it into the sale price you are paying.

It is not, thus, free money, at all.

On a macro level, that's true. On an individual level, your avacados cost 3% more whether you use credit or debit, it's just your choice to recover 2.5% from that back in rewards or not.

It can also be your choice to lobby your government to shut down the rent-seeking behaviour of Visa and MasterCard.

It can be done. The UK and the EU capped fees at 0.3% for credit cards and 0.2% for debit cards

How do you think they got that 2.5% to "give back"?