Really, tho? It was already Department of Justice policy that a president could not be prosecuted while in office. Every cop in the country enjoys qualified immunity, which amounts to more or less the same protection.
read the dissents! the immunity granted to the president is categorical in a way that qualified immunity is, well, qualified. If a president orders the military to go out and shoot people, he's immune from prosecution because "giving orders to the military" is his duty.