I concur it's probably either an outdated rule or mis-communication, but with the price of Duty Free here I like to believe it's a conspiracy (I took QF30 back from LHR last year and the Duty Free people there made me sign a disclaimer stating that if my alcohol was confiscated in transit they would not be responsible, as they do not allow Australia-bound flights to take DF Alcohol aboard; needless to say, the booze made it home just fine, after being told that we were welcome to leave items in the overhead lockers during transit, something i've done many times).
It would be nice if the powers-that-be of all the airports and respective aviation security agencies actually interacted with each other on a regular basis, and made rules that were coherent and consistent, but that's a utopia