I'm afrad your entirely correct rant is about 25 years too late.
"Duty-Free" shops were a dishonest rip-off when I set one up at a regional airport in 1986, and they still are.
Most items were not dutiable in the first place, and even if they were they were cheaper with the duty in any supermarket or high street shop in the nearest town.
If you have a delay, or otherwise have time to kill in a departure lounge, ask to see the shop manager and ask him/her what duty his/her stock - ostentatiously labelled as such - is free of. Initially you will be told a load of BS, and then the next stage is a discussion of the law relating to fraudulent descriptions of goods, etc etc. If you are in a BAA airport (ie my one-time employer) ask for the Terminal Duty Manager and have the same discussion.
Nothing will change, of course, but it passes the time.
I had no future in the BAA once the grocer took over the asylum. At meetings about extending the time passengers are forced to be in the departure lounge (increasing "dwell time"; a management KPI), it did not do to mutter that airports are to transfer people and goods from land to air as quickly and efficiently as possible, not to make it a protracted and unpleasant experience while also defrauding them by selling goods wrongly labelled as duty-free and/or cheap tat at inflated prices.