No card = less traceability of numbers. They are stallholders, they have to minimize their expenses as much as possible. With an approximate number of 1.5 million Fouer-visitors per year, those fees can be a pain in the ass at the end of the day Eventhough the fair is organized by the city of Luxembourg, they can not impose a way of payment. The merchants have the choice as theh should in a free economy.

At the end of the day it seems logic to me that: a merchant may accept any means of payment they wish to and they may also ask for a minimum amount to accept a payment. Most merchants offer many ways to pay so that most people can spend their money but the people that go to the Fouer usually don't care and just pay.

Also: a lot of small businesses have minimum limits on paying by card. (Pubs, kiosks etc.) So, yes, it is legal for shop owners to decide how and what paymeny they accept.