The way it REALLY works is that the merchant charge % varies per card. The higher paying cards have higher merchant fees. What this does is incentivize the card holder to get a card with even better payouts, which results in even higher merchant fees. Card holders with high perks are freeloading off those who use cards with fewer perks, or none like cash or debit card users.

It's a huge externality. If it were the law that merchants had to make the card holder pay their own fees, this whole perverted system would go away. The only way it works the way it does is because EMV contractually requires merchants to obfuscate the true cost from the consumer.