[kibibyte]

This wouldn't surprise me, given that Amex does have a consumer reputation for being very friendly for disputes. That factors in two ways: a successful dispute means that the merchant is completely out that money, and every dispute also costs something like $25 to handle regardless of whether the merchant ultimately wins or loses.

[matthewdgreen]

It's much easier just to model this as an additional merchant "fee." If a card tends to dump fraud liability onto the merchant (and here we're talking about fraud that the merchant really isn't in a position to prevent, but the card issuer/network might be) then this simply increases the cost born by the merchant who accepts the card. Of course it's extremely difficult to estimate these costs, which is probably a desirable feature for card issuers/networks, since lack of price transparency is usually helpful if one wishes to overcharge for things.