All day yesterday I listened to Trump partisans--lawyers and laypersons alike--cheer about Mueller's final act. But except for Alan Dershowitz, not one of them apparently grasped the poisonous seed the Special Counsel planted in his report.
Mueller said he could not exonerate Trump from a charge of obstructing justice.
Put aside whether obstruction can be laid against a public official when acting within the scope of his sworn duty (see the Rick Perry case), put aside whether that crime is applicable when a principal (Trump) fires his agent (Comey), put aside whether constitutionally there are any limitations on whom in the executive branch the President can fire (or why), put aside whether obstruction was within the jurisdiction granted to Mueller by Rosenstein (concerning collusion), and put aside several other thorny legal questions.
Even if Mueller was charged with investigating whether there was any factual basis to support a finding of obstruction, that would have been his only task. His job was not to "exonerate" Trump or anyone else. Mueller's sole job was to answer only one question: Were there any facts showing obstruction?
Mueller had the burden of proof. In gratuitously opining that he could not exonerate Trump he admitted his failure to carry his burden of proof because he admitted there were no facts to support it.
In planting that beyond-his-jurisdiction seed of doubt about obstruction, Mueller has handed Trump's enemies in the House and media the fertilizer to grow yet more investigations until they blossom into the full flower of an impeachment proceeding (if the democrats continue to be strategically and tactically tone deaf to the American people).
Mueller's obsession with obstruction of justice was ultra vires and unproved. No matter what weasel-words he used to help his Deep State allies.