One of the big problems many of these stores have is that the mobile coverage actually is bad inside due to the building's structure. Many of them are reinforced concrete buildings wrapped in metal cladding (take an IKEA store as an example). So many larger supermarkets in some countries barely have coverage at the cash register so you can pay with a phone but you'll lose coverage as soon as you enter further. They mostly don't offer any solution for this, like WiFi APs or mobile base stations. There's also the much higher density of people at that spot which crowd the same base station.
I'd wager MediaMarkt faced the same issue, wanted to give some internet coverage, and found WiFi to be cheaper or easier to implement and maintain. They may have realized in the meantime that they can extract some useful data too.