The strange deals on the entire future output are what was allowed. Try to do the same thing with onions and the government understands you are a criminal.

https://en.wikipedia.org/wiki/Onion_Futures_Act

That is quite the amusing read but it seems like a poorly constructed law. It wasn't futures themselves that were the problem there. The duo engaged in blatant market manipulation and severely disrupted part of the food supply in the process.