There was a documentary a year or 2 ago on Frankfurt and the shops, restaurants etc brought in a lot more money than the airline operations.
Though why pax feel the sudden urge to buy a £400 suitcase or a Rolex watch at the start of their journey I've no idea, especially given the exorbitant prices at airports.
Are pax so stupid as to believe that just because they are in a "duty-free" zone then things will be a bargain?
I remember going into Dixons at NCL a while ago and they wanted £120 for something when the high street price was £60.