In the USA, by law, they are not allowed to stick it to the customer for more than $50.
If they ate the charges, they are afraid that a lot of merchants would deliberately ring up fraudulent purchases for the guaranteed profit.
Those two facts force them to the current system. And the fact that merchants are not allowed to charge customers different rates for different cards gets rid of incentives for merchants to charge customers for the poor security practice that the credit cards have.
There clearly are merchants who do that today. Last time we have a card stolen, we got tons of random bullshit merchandise in the mail (weird cosmetics and such), presumably for the affiliate money.
You are clearly arguing from the perspective of a 'good' consumer.
But think about it for a second, those charges were inflicted on good merchants, the affiliates are not the merchants, they're in the same boat that you are in, except they lost their goods, the affiliate pay-out and a chargeback fine on top of that.
If they ate the charges, they are afraid that a lot of merchants would deliberately ring up fraudulent purchases for the guaranteed profit.
Those two facts force them to the current system. And the fact that merchants are not allowed to charge customers different rates for different cards gets rid of incentives for merchants to charge customers for the poor security practice that the credit cards have.