Shall we address the general question of privacy, or just close the particular breach that affected a congressman? You know it's going to be the latter.
It is a lot harder to corner the market when you have very non-perishable or very perishable goods and/or much larger domestic and global markets with harvests that vary across geography and time, like wheat and frozen orange juice concentrate. Most goods traded via futures have those properties. Onions is right on the edge of even making sense as a futures commodity, and because of that, it's a recipe for manipulation.
And there were calls to regulate potatoes the same way in the fallout of the onion market manipulation, but those didn't pass, and in 1976 the potato market was almost cornered in the same way.
But sure, govment bad.
As I wrote in a previous comment on this thread I am sure the congressman that passed the law discussed the issue with farmers and ranchers producing all traded commodities and the ones that were not growing onions did not think there was a significant risk of a corner and they preferred to have their goods traded.
Although I do agree with you that privacy protections should be broader. As far as Megan’s law I am sure you realize that it is intended to protect everyone from sex criminals not just people named Megan.
Legislative product management, by committee.
A direct democracy might have just banned all futures.