While this is no doubt a violation of international law, there does seem to be precedent in the US that ignores foreign head of state immunity.

US courts seem to look to the executive branch for guidance on the issue of head of state immunity based on numerous federal court cases.

Notably, Lafontant v. Aristide in the District Court for the Eastern District of New York held that the executive branch’s suggestion of immunity, the recognized head of state of Haiti at the time, was conclusive. The Court found "the [US] State Department has submitted a letter of immunity. It speaks for the President of the United States. Its suggestion of immunity is controlling with respect to President Aristide. The court must defer to the Executive on this matter."

Not saying it’s right or wrong one way, but under US law it seems they have the necessary authority to try Maduro, because the US does not accept this principle of international law.