Hi Pilatus
Because each AC flight is, in effect, a State flight, then the airport operator/fuel retailer charges them a non-duty Government rate or else, if they are charged the full rate, then the Government claims the VAT/Duty back later.Governments don't charge each other for fuel, as such, but merely exchange it.That is, 240 lands at an RAF base, takes up fuel,the RAF don't charge them.An RAF Sea King lands at Baldonnel, uplifts fuel, the Don doesn't charge them.The seperate Governments settle up at the end of each tax year, just to level the books, if they have to.
regards
GttC