When I visited China, the expats told me that recreational drug supplychains were strictly compartmentalized. There was the supply of illicit drugs for Westerners (imported by the sons of Nigerian businessmen, the cliche went), the supply of illicit drugs for Chinese people (who only dealt with Chinese people), and then there were the vast array of drugs that are completely legal to get over the counter in China without a prescription (at a pharmacy or CTM shop) that would be controlled substances in a US pharmacy.
That the official line from the CCP was that China had no drug problems, no prostitution, a variety of other things†, and that there were no gay people in China; That these were all Western ailments.
Urban China is a panopticon state not only digitally, but culturally. Housing is much tighter than the US, walls thinner. Your underwear is hung out to dry in clear view. "Privacy" in terms of politeness norms mostly consists of pretending you don't see or hear a thing. Neighbors generally know a lot about what each other are doing. 7% of the population are Party members, and in Marxist-Leninist systems this connotes something closer to earning a military officer commission; The Party is not trivial to apply to, the Party is strictly regimented, Party rules are held above and before the civil law, Party members are expected to be informers and have a strict lawful-good orientation from the perspective of the regime. Penalties for commerce in illicit drugs are even more extreme than the US, and due process is not bound by the same presumptions.
There are lots of factors conspiring against the sort of informal US inner city street drug distribution being as big of a deal in China.
Disclaimer: All my information is more than a decade out of date, and was only ever a thin slice of opinions from mostly Westerners in some first tier cities.
† From an academic paper: "2 The Six Evils are prostitution, drugs, selling women and children, pornography, gambling, and feudal superstition. Criminal gangs, or triads, are often counted as a seventh evil. These vices represent impediments to modernization and throwbacks to social problems that were present prior to the Communist takeover. Elevation of a problem to an "evil" symbolizes that the Beijing regime will mount a "campaign" or "struggle" against it."