Don't forget, it is not just the shops. On board sales on flights to cities outside the EU have exactly the same prices as flights within the EU. They too are ripping you off.
Some airlines offer duty-free sales on some goods on these flights, but the cost of buy-on-board remains the same.
I don't accept the argument duel pricing is too difficult. Simples, take 16.6% off the gross price. My phone has got a calculator and I can figure that out in a second.