When are U.S.-bound flights required to confiscate liquids at the gate?

In a recent question, I found that the U.S. requires flights from Hong Kong that are bound for the U.S. to confiscate liquids in containers above 100 mL from carry-on luggage in a secondary screening at the gate. However, this did not occur on a recent (June 2014) flight I took to the U.S. from Incheon, South Korea. I've also read forum posts where this has not applied to others going to the U.S. from Seoul or parts of Europe, but also where it has applied to those flying to the U.S. from some other places, including Latin America.

So, under what conditions are U.S.-bound flights required to confiscate liquids from carry-on items at the gate (as opposed to only confiscating them at security checkpoints and allowing items purchased airside to be taken on the plane, as happens on U.S. domestic flights?)

Is this only from certain countries whose primary screening isn't approved by the U.S. DoT (or DHS, as the case may be) for some reason? If so, is there a definitive list of which countries require (or, conversely, don't require) this check at the gate?

Is there an element of randomness to whether liquids acquired airside may be allowed on any given U.S.-bound international flight or is the procedure generally the same from a given departure point? It appears that, at least in the specific case of Hong Kong, the gate check is always applied, but I'm wondering about the general case of U.S.-bound international flights.

In order to limit the scope of the question, I'm not referring to items purchased in duty-free shops and the special rules and circumstances that may be involved with those. I'm just concerned with filling up or buying a water bottle or buying a soda or some such thing airside and taking it on the plane.