It's a combination from my observations.
They buy duty free and then drink it before the flight.
They buy (not so) cheap beer early in the day (cheap holiday flights operate at odd hours) when it hits them harder having not had breakfast etc)
I think the combination of staff restricting sale to clearly drunk people. Airport security being able to reject flight to drunk people and duty free being delivered to the gate would help. Oh and being able to ban people from future flight if they breach the rule.
Golly I am having an evil morning.
