When I buy duty free in Stavanger, I'm always asked if I've got an onward flight, so I wouldn't buy it if I did.
Rules are rules, and are plain and obvious for all to see. I have some (quite a lot actually) sympathy for the occasional flyer, but people who fly regularly or read these forums know the current situation, whether or not they agree with it, so shouldn't attempt to take liquids through security. (Although I'm unclear whether the plastic bag should be taken out or your hand baggage for screening).
Finally, I've never had a bad experience with a security numpty, I work on the principle that if you are nice and friendly to them, they will be nice and friendly back (actually, 25 years ago there was an immigration officail in Libya, who was a total prat when the immigration card was updated, but the translation wasn't, and just ripped it up when it was filled in wrongly and sent you back for another card)