This is another anomally of the security regimes throughout europe, I regularly transit through Bergen(for a domestic destination) and on arrival there you can buy duty free and continue on your way however I had not bought any until the very helpful duty free guy pointed out that if I wanted some and wished to continue to the aforementioned domestic destination they would simple seal the goods in a transparent bag with the receipt showing, I was a bit dubious at first but it works, a very sensible and common sense approach if you ask me.