Mueller's report explains his prosecutorial decisions in four points.
- The DOJ's OLC has issued the opinion that a sitting President cannot be indicted, and the SC accepted that opinion for the purposes of his investigation, further recognizing that a DOJ indictment might preempt the constitutional mechanism of impeachment.
- The investigation was nevertheless warranted because an criminal investigation is permitted under the OLC's standard, even when an indictment is not. Other individuals engaging in obstruction could be prosecuted immediately. And the President is not immune from prosecution after leaving office, regardless of whether impeachment proceedings are brought or are successful. So the investigation served the purpose of preserving evidence while witness memories were fresh.
- The normal public mechanism for an individual accused of a crime to clear themselves is a speedy public criminal trial. If the sitting President cannot be brought to a criminal trial while in office, then it would be unfair for the SC to affirmatively accuse him of a crime that cannot be prosecuted in a criminal court of law at this time. Even a sealed indictment's secrecy could not be guaranteed to be preserved. Accordingly, a criminal accusation against a sitting President could be harmful to the country, because the accusation cannot be resolved in the normal adversarial manner of a criminal trial.
- The results of the investigation do not allow the SC to conclude that the President did not commit obstruction.
So please explain how this exonerates Donald?
Quite obviously, it doesn't exonerate Trump.
Quite obviously, Mueller was desperate not to necessitate Trump to be indicted. The reasons for that are probably complex, having to do with a sitting President not indictable under DoJ guidelines, and a Constitutional crisis in case an indictment is pending, since this breaks the rule of law.
In order to accomplish this, Mueller had to make a few stark, almost scurrilous decisions.
1. He indicted no one in the innermost Trump circle. That would, almost by necessity, require Trump also to be indicted, at least as a co-conspirator. And that's why the Trump Tower meeting, despite being quite obviously a blatant violation of the law (no thing of value be accepted from a foreigner) on the even more scurrilous contention the participants cannot possibly have known it was a crime, even though they lied and lied about it, including Trump himself.
2. He chose not to follow the evidentiary standard of Grand Juries (probable cause), but rather inexplicably the standard of a jury trial (beyond reasonable doubt).
3. He chose to almost ignore the amassed evidence for Trump and the GRU working together, and focused on an allegedly required "agreement" necessary to prove criminal conspiracy. So, not having been able to unearth such an agreement, the whole Volume I detailing collusion remained without consequences.
4. The same as above happened to Volume II and the detailed evidence for obstruction, not to mention witness tampering and suborning perjury. There is even a section arguing that the established pattern of obstruction is itself evidence for corrupt intent, but still, the whole thing remained without the obviously inevitable prosecutorial decision, namely, indict the crook.
5. He chose not to pursue in earnest a personal interview, and not to subpoena him, knowing full well that there's a guarantee the off-the-leash blowhard would perjure himself.
So, in a way, Mueller, following DoJ guidelines like the good soldier he is, suffers Trump crowing about "complete exoneration", while his mindless minions even take it a step further crowing "no there there, no exoneration necessary." I cannot begin to imagine how much this has to grate the old man. A DoJ guideline rendering the President above the law forced him to make a joke out of his own investigation, and to do immeasurable damage to the rule of law itself.
There is just one single saving grace in this (which he also explained), that is, to preserve the evidence for the Constitutionally created powers, namely Congress, to go about their Constitutionally mandated work. It is way past time they start in earnest.