I've been looking into this myself for the past few days because I'm interested in the Norinco NP-34 and I wanted to figure out if there was a specific reason they weren't allowed to be imported into the US.
Here's what I found:
1993 Clinton administration blocks importation of Norinco (most likely all Chinese firearms) into the US.
1994-1996 "The largest seizure of fully operational automatic weapons in U.S. history" Chinese Arms dealers including at least 3 Norinco Representatives broker a deal to sell 2,000 full auto AK variants, weapons parts and grenades to what they believe are Organized Crime members and Drug Dealers. After the shipment is received, they offer shoulder fired rockets,mortars and anti-aircraft missiles to the customers...turns out they're BATF & Customs Officers!
2003 US imposes sanctions on Norinco for allegedly selling missile related goods to Iran. This leads to total prohibition on the importation of Norinco products into the US.
Sooo.....yeah, that's what I found, take it for what it's worth. It's kinda too bad because I really was interested in the NP-34.