FTZ's were originally created to provide relief to companies that were inadvertently negatively impacted by tariffs imposed by the US.
Materials that flow into/out of FTZs are highly regulated by CBP. Once an FTZ site is activated, the property owner has agreed to allow CBP to make unannounced inspections of the FTZ 24/7/365. It is highly unlikely that FTZs are utilized as an ongoing tool for trafficking considering CBP's involvement in the program.