In the UK it is quite common to not accept AMEX and other cards such as whatever 'Maestro' is. Therefore you do have to show what you do accept so people know to use a sensible-to-the-retailer card.
Maestro is huge in Germany. Many people do not have CC cards, but every singe adult has an EC / Maestro card. And we are really annoyed when they don't work abroad (though they do work with ATMs worldwide in most cases).
Maestro used to be Switch. They were popular at one time for under 18 year olds for some reason. AMEX is really seen as a premium option here in the UK, for travellers and those with rich blood.
Maestro is in effect a card network, the under 18 popularity stemmed from having a line of cards that were low transactionality, it was the first issuer in the UK to really give cards to under 18's.
