Mueller deduces that the guidance (a) prohibits indictment in order to avoid a public charge that would undermine the capacity of a president to govern, but (b) permits investigation with an eye toward post-presidency prosecution. The special counsel pretends that this gives him “fairness concerns” over the president’s due-process rights: If, after a thorough investigation, a prosecutor made a judgment that the president had committed a crime but did not charge him, Mueller reasons that the poor president would bear all the stigma of a criminal accusation but would have no opportunity to clear his name in formal court proceedings. That is, the OLC guidance denies him his day in court.
Mind you: Mueller says this as a précis to pouring out over 200 pages’ worth of obstruction evidence — and, implying that this evidence is quite serious indeed, he is at pains to tell you he will not “exonerate” the president, even though he hasn’t charged him. That is, Mueller’s report is designed to taint the president when he does not have the constitutional protections of a criminal defendant — exactly the thing Mueller claimed to be avoiding by not making a decision on obstruction.
Of course, there would be no such danger if the report had been kept confidential, as federal regulations require. There would be no such danger if Mueller had simply done his job, made the required binary decision about whether or not the evidence supported indictment, and left the application of the OLC guidance to the attorney general.