The provisions of the original FTZ have evolved to the point where Foreign Traders has established the equivalent of Mexican maquiladores with our borders allowing huge benefits to foreign corporations.
For a list of FTZ see the following link.
Enforcement - U.S. FOREIGN-TRADE ZONES
For more detaile information. of history, duty and tariff avoidances, and operation details, see the following Wiki article
Good example. FTZs are so lucrative because they are bubbles of lowered Government burden.
Imagine what a proper economic freedom zone would be like. Right to work, no Corporation tax, no minimum wage, no Obamacare, a defanged EPA, etc etc.
Right now the only places that allow that kind of economic freedom are other countries. A government-free zone in the US would blossom like a hundred Hong Kongs.