In France, the fees required to accept Amex are still way higher than MC/VISA. They are still not widely accepted and not a lot of persons know about Amex.
High-end businesses accept Amex, because it's worth it. Major chains can probably negotiate good deals. Tiny/seasonal businesses often use middlemen like Zettle that charge high fees and accept almost every payment method imaginable. Those in the middle who use traditional payment terminals and pay list prices may still avoid Amex due to the high fees.
Visa's highest tier cards (Visa Infinite cards, such as the Chase Sapphire Reserve), already have higher fees than the Amex Platinum. The difference is, Visa won't let merchants ban single cards (you have to accept ALL Visa cards), while banning Amex meaning you are banning mostly high tier cards and losing nothing on the low end.
With Amex, Amex themselves are both the issuer and the acquirer, so there is no interchange to regulate.
https://curia.europa.eu/jcms/upload/docs/application/pdf/201...