Most of the above are right and I'm not going to add much in detail but
Boarding card shows destination - Domestic, EU or non-EU which is then linked back to transaction for VAT analysis
Retailers can use agregated data for their own marketing analysis but not in great detail i.e. can't see that pax A travelling to destination B bought items XYZ but can see that x% of pax travelling to Greece bought tobacco.
WHS do premium price vs High St - same with confectionery, soft drinks etc even though they get away with not paying VAT/Duty on a number of transactions. If you saw the rent they're charged you'd understand why. Re Tobacco in WHS I seem to remember there's a clause that anyone buying single packets is likely to be for consumption in the country (even though under new regs you can't smoke in the airport
) therefore are not considered to be 'Duty Free'
In summary there's no black & white - just a whole load of shades of grey that retailers interpret in different ways.