Their payment processor (the people they rent the machine off of) offers them this oppurtunity to 'unlock hidden revenue for merchants'[1][2][3] and they are happy to do this.
Visa in fact tried to ban it: https://en.wikipedia.org/wiki/Dynamic_currency_conversion
Of course, there are regulations and agreements with various institutions that should be followed - but it's free money for the shop, nothing else.
[1] https://www.shift4.com/blog/dynamic-currency-conversion-unlo...
[2] https://www.fexco.com/payments-and-fx/currency-conversion-so...
[3] https://docs.adyen.com/point-of-sale/currency-conversion/