<<Amex will let you dispute anything, for any (reasonable) cause, at least temporarily until the dispute clears of course. I was told by the Chairman folks that they had a checklist that they had to go through to determine whether or not to open a dispute,>>

I wonder if this may reflect differences in the merchant agreements used by the American Express versus the Mastercard network. The only way I can think to test would be to dispute a charge on a Citibank American Express card.