If you fly an internal flight, then I would assume the shop has to pay a duty on the goods sold, right?* How they advertise it, or whether the buyer is getting a legitimate savings compared to your best local liquor-shopping option, is another story.

*Or do airport shops strike deals with their local governments to not pay taxes, even when the shoppers aren't leaving the EU? I've never fully understood how this arrangement works, only that a 1 litre of Scotch at LHR generally costs around the same as a comparable 750 mL bottle in Kansas; thus I'm a buyer. I've never paid attention to whether intra-EU shops are any different.