I once had a conversation with a merchant about not taking AmEx. He told me they could take it, their machine and merchant accounts were all set up, but instead of getting paid [whatever they charged] - [small percentage merchant fee] relatively quickly, they were sent a bill for AmEx fees that was almost impossible to correlate with sales. AmEx only paid them for the sales later.

AmEx are 'better' for customers because they screw the merchants over.