Smart Republicans are ignoring, Trump is likely incapable of understanding (and if he was capable, he would deny it and lie about it), and even some responsible journalists and media personalities are finding it difficult to understand or articulate exactly what was Robert Mueller's justification for not "coming to a conclusion" on obstruction.
A careful reading of the report (which I admit I have not yet completed, but am depending on Rachel Maddow and others at MSNBC to pick out the critical passages), describe the Mueller team's approach as an almost scientifically objective one, "...we determined not to apply an approach that could potentially result in a judgment that the President committed crimes."
Why? Mueller cites in detail relevant Justice Department rules from the manual about not being allowed to indict a sitting president, and given that the unfairness of even suggesting a criminality when there was no courtroom or process for mounting a defense (apparently the bully pulpit and the court of public opinion were not considered adequately objective).
So does that mean he left it to Barr and Rosenstein? That would make no sense. They are limited by the same rules Mueller is. Their decision to absolve the president of obstruction charges, even if based on a disagreement with Mueller's justification for not coming to a conclusion, is revealed as political by the way that Barr made the announcement weeks before releasing the report and prefacing it's release with a press conference with an audience of one who wanted him publically to say "no collusion."
Someday, Rosenstein may ask the American people for a mea culpa and talk about how conflicted he felt as he stood behind Barr as he lied about the report, stoically looking straight ahead and keeping his face in a tight mask. Though he is a hero for protecting the Mueller investigation, he has also shown himself to be a company man and a coward twice--once when he did not speak publicly about Trump's lie pinning the firing on his memo, and again when he allowed Barr to persuade him to stand behind his decision to absolve Trump of obstruction.
But back to my original thought--it is very hard for anyone to understand the degree of objectivity and restraint Mueller and his team used in avoiding even seeming to pass judgement. It is a type of thinking they share with scientists--the ability to withhold judgement and simply reveal the facts. I guess in a way, it's like the legendary Dragnet character, Sgt. Friday, who used to say something like, "Just the facts, ma'am. All we want are the facts. All we know are the facts."