Perhaps this explains the rise in American Express's stock price today. Amex has perhaps the best customer service I've experienced (maybe that says more about me than them :).

While Visa is a card network, Amex is both a network and an issuer. Visa works with issuing banks like Chase, so as a Visa cardholder, you would interface with the bank rather than with Visa when reporting an unrecognized charge.

Given that Amex is an issuer, I could imagine that there might exist more incentives to side with the cardholder versus a merchant for disputes, as compared with a company that operates as a network but not directly as a card issuer.