This is by no means unique to Argentina.
Post-security liquid purchases have been confiscated in Costa Rica, Mexico, and Hong Kong... to name just a few.
In my experience, the private security firm that American Airlines uses in Buenos Aires is much more aggressive than the United/Continental side. They're usually incredibly pleasant; in fact, they're some of the kindest I've come across in the world
It used to be that duty free liquids were not allowed on flights from Israel to the United States, but that has been relaxed. They used to hand write "no liquids" on your boarding card.
Now, at least, you can bring duty free liquids through TSA checkpoints. It was always a problem. If you were connecting in the US, the announcements said that you weren't allowed to open your checked luggage in the customs hall. So what were you supposed to do with your duty free liquid purchases or airline amenity kits?
The amenity kit still has to be stashed in checked baggage, but the duty free is allowed as long as the seal is intact.