Fraudulent and colluding merchants are a huge part of the fraud problem. You mean to suggest that in a perfect world, Amex wouldn't stick it to good-faith merchants. But for the most part, there are only good-faith customers, so they clearly aren't eating charges, and having Amex eat charges raises price for everyone.
The only party that has access to the information required to do proper scrubbing are the issuers and the banks.
The bona-fide merchants have to make decisions about charges being fraudulent or not in the absence of this information.
If they guess wrong they end up paying or lose a sale.
Malafide merchants don't care one bit, they'll be up and running under a new identity next week, so they never get stuck with these charges, the simply fold and play it again, and in that case the card issuers do eat the chargebacks.
A bona-fide merchant is a sitting duck, and trust me, this comes to a lot of money on an annual basis. Consumer fraud exists and it is a serious problem.
