It’s a simple fact of government investigations: If a company cooperates with prosecutors and gives them the information they want, the prosecutors remember that when the time comes to decide on indictments, sentences, fines, or other penalties.

That gives companies powerful incentive to procure as much information about possible wrongdoings as possible, including information gleaned from interviews with employees—who may well put themselves in personal legal jeopardy by answering the company’s questions.

Enter the Upjohn Warning.

Also known as “corporate Miranda warnings,” corporate lawyers invoke the warning to remind employees that the lawyer works in the best interests of the company, not ...