Pandering, or "offering or promoting prostitution" is still a crime.
The general pattern for drug and prostitution stings is that the police will commit those secondary crimes, but intervene before allowing the primary crime to happen. In this case, they pretty much committed the primary crime at length before acting.
In a way [0], the US does preemptively ban chemicals, regardless of whether that chemical has even been synthesized or used recreationally yet.
We don't criminalize baking soda, we're just trying a Minority Report sort of approach to things that are likely to be used rather than playing catch-up with known-psychoactive analogues. Certainly the metaphor bit still applies - selling and importing these things are only banned because of their abuse potential, not because the chemical is seen as intrinsically bad.
But I stand by the point of the metaphor, which is that we criminalize distribution because of use. If you couldn't use it, it wouldn't be illegal. So even when flipping users, the intent is to run stings without letting the primary 'event' of drug use happen. That seems like the key difference between drug stings and this sting.