From Wikipedia[0] because I can't be bothered to read more than a few paragraphs:

> In 1909, well before the Securities Exchange Act was passed, the United States Supreme Court ruled that a corporate director who bought that company's stock when he knew the stock's price was about to increase committed fraud by buying but not disclosing his inside information.

Based on anti-fraud common law alone the court decided it was illegal for an insider to trade stocks with non-public information. An explicit law would be nice, but a reasonable interpretation of basic law would see most of our ruling class behind bars. This is only highly-contested and technical because we've let our standards slip so far.

0: https://en.wikipedia.org/wiki/Insider_trading

If it doesn’t then it should.