Originally Posted by NickW
I think this comment rather misses the point that the original poster was making.
Yes, some major retailers don't take Amex and don't claim to. What the original poster was referring to was the case where a large retailer would claim to accept Amex and then refuse at the point of sale.
This is unusual and I have never seen it happen in the UK.
I do not think the original poster made this point though - do Sainsburys in France have an agreement with Amex to take the card? If they do, then it was wrong to reject it.
Just because in the UK it is accepted at its stores, it does not mean that it will be accepted in other countries.
It is clearly up to the company to decide.