Honestly I just don't think the conservative Justices are that smart. Here's Roberts arguing that because something never happened (criminal prosecution of a former president) no one could have reasonably assumed it would happen:
> Unable to muster any meaningful textual or historical support, the principal dissent suggests that there is an “established understanding” that “former Presidents are answerable to the criminal law for their official acts.” Post, at 9. Conspicuously absent is mention of the fact that since the founding, no President has ever faced criminal charges—let alone for his conduct in office. And accordingly no court has ever been faced with the question of a President’s immunity from prosecution. All that our Nation’s practice establishes on the subject is silence.
Literally on the next page, here's Roberts arguing that though something has never happened (criminal prosecution of a former president) it is very likely to happen:
> The dissents overlook the more likely prospect of an Executive Branch that cannibalizes itself, with each successive President free to prosecute his predecessors, yet unable to boldly and fearlessly carry out his duties for fear that he may be next. For instance, Section 371—which has been charged in this case—is a broadly worded criminal statute that can cover “ ‘any conspiracy for the purpose of impairing, obstructing or defeating the lawful function of any department of Government.’ ” United States v. Johnson, 383 U. S. 169, 172 (1966) (quoting Haas v. Henkel, 216 U. S. 462, 479 (1910)). Virtually every President is criticized for insufficiently enforcing some aspect of federal law (such as drug, gun, immigration, or environmental laws). An enterprising prosecutor in a new administration may assert that a previous President violated that broad statute. Without immunity, such types of prosecutions of ex-Presidents could quickly become routine. The enfeebling of the Presidency and our Government that would result from such a cycle of factional strife is exactly what the Framers intended to avoid.
Just like, full on embarrassing. These guys need better clerks or something.