The Hunt brothers did not do anything illegal ... they simply risked their resources - either directly or leveraged - in a commodity play. The Carter administration just didn't like anyone else encroaching on their turf.
http://www.investopedia.com/articles/optioninvestor/09/silver-thursday-hunt-brothers.asp

The essential American soul is hard, isolate, stoic, and a killer. It has never yet melted. ~ D.H. Lawrence