Sorry to bump this slightly old thread but I was re-thinking this whole issue a bit more. There's a lot of answers here but I think a lot are not even all that correct. The 3-1-1 issue doesn't seem to be relevant at all because at practically every US airport, they sell water or drinks in the sterile area where you can bring onboard the plane (not to mention fill up your water bottle).

I suspect the real issue is the TSA and HKG authorities evaluated the main screening checkpoints and they were found to be deficient for US-bound flights. As a result either HKG had to upgrade their security (and make it more of a PITA for all passengers) or have US-focused security where they have enhanced screening. The same seems to happen at TPE where US-bound flights have gate-side extra screening/searches.

IIRC they used to do something like this at PVG years back but I haven't encountered it for a long time. It's probably because Chinese security is pretty invasive (I get the most thorough patdowns that put the TSA to shame at airport security).

Bottom line is for US-bound flights you need to meet some level of security screening standards the TSA has prescribed. Either it's applied across the board for the entire airport, or they must at least make sure the US flights are subject to that. It's just unfortunate it means you can't bring ANY water onboard. That's annoying if you're flying in Y where having a pre-filled bottle would be nice for a 12-15 hr flight.