Look at what happened the other day; Trump tweeted that he was going to cancel the contract with Boeing to build a new presidential airplane, and immediately Boeing stock values plunged. And it is Boeing, so of course, the share price will recover. Anyone who bought shares when they tumbled could then sell them once the price rebounded, and make millions.
Politicians, in other words, are in a position basically to manipulate stock prices. Obama or his friends and associates, can, for instance, buy shares of Raytheon today, and announce airstrikes in Syria tomorrow, and make astronomical profits instantly.