Airport Retail - the nonsense

Can anyone tell me why I have to show my boarding pass to buy anything at a UK airport - and indeed at many other airports around the world?

... and I don't want the palpably untrue explanations I get from the sales assistants about security or because of tax free concessions - I'm talking about why I have to show my boarding pass to buy magazines or suncream.

I suspect it's a data gathering initiative run by either the store or airport owners or both! And if my suspicions are true - I want to know if it is legal to demand this type and quantity of information purely for marketing purposes?

And if it is legal, it's certainly a hassle when I'm enjoying a coffee before flying and my kids want to pop off to buy some sweets at the newsagents - kids in possession of really important documents (boarding passes) don't mix well in time-sensitive, document critical, situations - telling the boarding gate official my children promised they wouldn't loose their boarding pass just doesn't cut-it I suspect!