Qantas have had some bad incidents recently with a passengers drinking too much duty free on the plane. There was the drunken grandma on a flight from Melbourne that had to turn around and go back.

Jetstar have also had some bad cases of passengers drinking too much as well. There was one passenger who started urinating on others.

I think if the FA is stopping passengers drinking their own duty free it might be due to airline policy rather than licensing laws.