Interesting. I have a PVG-LAX flight in August but can't recall having been subjected to such a "rule" on any flights ex-China I've taken in the past, though never really found any Duty Free bargains at those airports. I do recall this being invoked on a nonstop from JNB-IAD on SA a few years back. Was most disappointed because they had a very unique offering of Bowmore I'd never seen anywhere else and would really have wanted a bottle.

And as noted, a one's arrival airport in the US, if connecting with checked bags, the Duty Free can be packed away in the Customs Hall for the onward flight(s).

I have had water confiscated at BKK at the gate screening of a US-bound flight, utterly stupid since the water came from an airside source.