It also reflects common practice: https://oxcon.ouplaw.com/display/10.1093/law-mpeccol/law-mpe... (“ 3 Functional immunity applies to both sitting and former heads of state; however, this immunity is available to such individuals solely with respect to acts performed in their official capacity (Fox 667).”)

France, for example, has prosecuted former presidents for things like bribery. But I don’t think you can prosecute a former president of France for something he told the Ministry of Justice to do.

I suspect this distinction between official and unofficial acts is also what most people assumed the law was, until Trump forced the Supreme Court to confront it under a particularly unsympathetic set of facts.