In Poland AFAIK when you sign up for a card terminal the company that lends it to you usually gets a cut from the transactions (besides the bank and card companies).

And there is a lower % for normal/popular cards (Visa/Mastercard) and higher (even 4%) for Amex, so most stores just ignore it and remove the Amex symbol from their terminal.

And yet, my company provides Amex corporate card for bussiness travel, but we usually go to US, so Amex is more accepted there.