Wod.
I suspect there is legislation and/or international conventions relating to the rights of passengers to purchase duty free goods on ships and aircraft in international waters and international airspace.
It would probably be easier for Australia to restrict duty free sales within it's territory, than in international waters or airspace.
And if your concern is inebriated passengers, what is to stop a passenger boarding the aircraft inebriated, or getting a "full charge" or buying duty free at an intermediate stop? I can't see Changi Airport restricting Australian passengers to the fruit juice bar!
The Australian Duty Free Sales industry employs a lot of people and probably generates significant profits, which are taxed as company profits.
I think you would need to make a much stronger case for anyone to take seriously any denial of duty free sales to international passengers?
Tail Wheel