The classification of NAC is a bit more nuanced than that. In IV and inhaled forms it has been a prescription drug for many years. The oral form has been sold as a nutritional supplement over the counter. The FDA didn't "ban" it exactly, what they did was send letters to companies saying that NAC does not meet the legal definition of a supplement because it was approved as a drug in 1963; according to federal law, any substance approved as a drug before it was sold as a supplement cannot be sold as a supplement. . Amazon responded by "banning" it for sale on Amazon.
The FDA's whole rationale is pretty shady. In answer to a FOI request for documentation, the FDA produced a hand written note from 1963 saying that inhaled NAC was a drug. That's it!
But the status of NAC is still very much up in the air. The FDA is considering two requests to reconsider their "decision."
What makes this all the more ludicrous is that NAC is used as an antidote for Acetaminophen. The FDA has no problems with people buying a liver toxin over the counter. But the antidote for it? Noooo we can't have that!