Insider trading consists of an individual who is an insider to a company that is publicly traded, trading stock in that company on information they obtained as an insider.

> When Warren Buffett decides that he wants to buy stock in a company, he knows that if this became public, the target company's stock would go up.

Warren Buffett is not a publicly traded company, and in this hypothetical he is buying stock in another company, which (by assumption) he has no insider information about.

> Insider trading law in the US is about breaching fiduciary duty.

This is false. It's about protecting traders. This is why it applies specifically to publicly traded companies. If it was about protecting shareholders (from what?) then it would apply to all companies.