I thought everyone knew this. Obviously the shops weren't paying VAT, and they were just making higher profits/paying higher rent. Try not to need things at the airport, and if you plan to buy something there, check it's cheaper than your alternatives.
I once asked in Boots why the needed my boarding card and was told it was for marketing reasons. So, not only did they trouser VAT I should not have been charged for they lied to me.
As someone else suggested, I hope some smart lawyer screws these companies over.