The cynic says it's to tweak silly American rules. The justifiable reason is that they do a worse job screening the liquids coming into the airside area.

It's easy enough to get away with - saying "no liquids" to the checkers usually does the trick.

FWIW, PEK security has almost always been quick and efficient. There are multiple checkpoints, but most are emigration related. The metal detectors are sensitive, but the pat down afterwards might as well be a massage. Only time it was slow for me was when I went in the "express" lane, which was the exact opposite.