Why was the law against onion futures passed in the first place? Couldn’t the same scheme be done with garlic, or sweet potatoes, or turnips or something?

Because American lawmakers are weirdly specific brainless creatures. This is how you get all those laws named after individuals like Megan, and my other specific fave, the Video Rental Records Privacy Act.

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.

Because Vincent Kosuga.

A guy cornered the market in the fifties, caused a scandal, they made a special exception.