Exactly: it is a very simple situation - we collectively as the society decided that we do not want garbage peddled as medicine. FDA established how we determine "not garbage". Full stop.
Edit: yes, based on our current position Export from US->Other Country->Import to US for medicine violates "Not Garbage" state. It is no different than importing USB chargers that sell in China for $1.02. We do not want the vast majority of the population engaging in this though those who know how to are allowed to do it themselves.