You have to think of credit card fees as part of your customer acquisition cost. You should be willing to pay more if the customer is worth more to you. People and businesses with Amex cards are more affluent than average, so you are willing to pay Amex more to acquire them than you'd pay Visa or MasterCard. That's why Amex can and does charge you more, not because they're simply passing through costs. If that was the case, more businesses would choose not to accept Amex; many of those where customer acquisition isn't an issue already don't (like utilities).
Amex has more affluent customers, on average, because they don't give cards to people with poor credit, and most of their cards have a significant annual fee. Their customers are those who can afford to pay extra for more benefits, which happens to correlate highly with overall affluence.
I have a Platinum American Express. I pay $495/year for my card, plus $175/year for three additional cards on the same account. I spend anywhere between $350K-$500K/year on the card (both personal and business-reimbursed expenses).
Are my charges more expensive for you as a business to process? Probably. I'm also more likely to spend a lot more with you than your average customer.
Honestly? I probably wouldn't. I'm in the minority though. Think how many Green charge card, OPEN card, regular consumer Amex credit card, and corporate card customers withe Amex there are out there. Would they quibble over an additional 1%? Or would they shop elsewhere with someone who absorbs the cost?
But at least merchants will have the choice now. In markets where you're operating on a 3% margin whether they're using amex or not can make or break the deal. Giving the consumer the option to pay a little extra or choose another payment method seems like a better solution than simply blocking amex altogether
Amex has more affluent customers, on average, because they don't give cards to people with poor credit, and most of their cards have a significant annual fee. Their customers are those who can afford to pay extra for more benefits, which happens to correlate highly with overall affluence.