Without knowing all the details - I'm not a laywer - I can say that SEPA regulates IBAN discrimination too, and that within SEPA, it should be enough to have an IBAN from any SEPA member country to make payments - whatever that payment is for.

So demanding a French IBAN number (aka bank account) or even a French IBAN from a subset of banks would be at least a violation there.

Ofcourse if AMEX just requires a bank account from a brick and mortar bank in France to validate your request to get the card, and if approved you're allowed to pay off the card from any bank account in SEPA, then I don't see how it'd violate anything.