Amex is the Apple of the payment world -- they control and brand the whole thing end to end, and own the high end, charging a slight premium for some things when the market will bear it (and were both far larger premiums and lower volumes in the past).
With Visa/MC, it's a card association (until recently, a non-profit), and an issuing bank. The issuing bank may have a bunch of banking relationships with you (so you're unlikely to walk over a credit card), or only a credit card (so you're not that valuable to them anyway), and doesn't have as much incentive to protect the card association brand. End user consumers almost never contact the card association itself.
With Visa/MC, it's a card association (until recently, a non-profit), and an issuing bank. The issuing bank may have a bunch of banking relationships with you (so you're unlikely to walk over a credit card), or only a credit card (so you're not that valuable to them anyway), and doesn't have as much incentive to protect the card association brand. End user consumers almost never contact the card association itself.