Imagine for a moment that you are president of the United States and you are given an intelligence briefing with information that is vital to an allied nation's safety. Under no circumstances are you to share this information with anyone. You are then in a candid two-hour conversation with a jovial diplomat from a nation hostile to that other nation. If you didn't keep your wits about you, you might accidentally discuss the secret information.
Donald Trump is not a man who has ever had his wits about him.
They were basically illegal until 1982. It was considered, and I believe still is, market manipulation.