that's the whole point of duty free. The Tax duty on those goods does not have to be paid by persons living outside the Tax area (such as those going outside the EU).
However, there are restrictions on what you are permitted to buy so you don't bankrupt the country you are buying it in and also so you don't have import issues at the other end.
Airports however, charge duty free shops loads as it is an unregulated income for the airport. And the airport has to make money somehow to pay for all the services it provides.
And thanks to low cost airlines (and it has to be said, dimwit commercial managers at airports), most of the cost of running the airport is passed on to the travelling public through things like car park fees, income from duty free and other shops and various non aviation related areas.
So you want to pay no more than £100 to fly somewhere and then baulk at the £250 car park fees.
Of course, you could pay £350 to fly and get free parking. (But the low cost airlines won't get passengers, as unfortunately there target customer is those who cannot afford to fly, and the legacy carriers will just get greater yield from those who can, so you still pay the £250 parking fee, courtesy of the dim-witted commercial manager)
Heathrows landing charges are set by the CAA. However, they can charge what they want for non aviation related income streams. so they do.
Other airports are not regulated in cost charges so they can squeeze the airline market as best they can. So, stupidly, they jump at getting low costers in, who shuffle hundreds of cost conscious persons through their terminals and hope they can tip you upside down and empty your pockets to pay for the bits the planes need to land on etc.
The tip here. Know your prices before going into a duty free shop.
(Example. Oban Whisky in Oban distillery is £42 or was last time I bought some. £35 in Heathrow whisky shop - £53 in Abu Dhabi airport whisky shop).
(1ltr of Jack Daniels in Guernsey duty free is £14.99 by the way
)
Airports make nothing out of me when I go through them. save 100% everytime.