I had always assumed that as passengers we were legally obliged to hand over our boarding passes when shopping in UK airports.
However we're not and the stores are using it as a way of claiming VAT back from HMCR without passing those savings on to consumers.
Don't know why I'm shocked but I am
www.telegraph.co.uk/travel/travelnews/11794109/The-real-reason-airport-shops-want-to-see-your-boarding-pass.html