The shops ask to see the boarding pass (or boarding card) to check the passenger's shopping rights.
Due to the Finnish tax and customs legislation, the Tax Free and Duty Free Shops selling tax and duty free products have to ask to see the passenger's boarding pass. This way the cashier can ensure that the passenger will take an international flight and is entitled to duty free purchases.
Read more about the passenger's shopping rights at the airport
Welcome to shop at the airport!