The following is an excerpt from a Politico article about the surprises in the Mueller report.
I think that one of the most surprising parts of the report was its discussion of why it could not reach definite conclusions on obstruction of justice.
Barr concluded on the facts and the law that DOJ could not charge Trump with obstruction. He and Mueller clearly were bound by past DOJ opinion that a sitting president cannot be indicted.
But even if a president lost that immunity, Mueller could not indict on obstruction because he a) chose not to seek a live interview under subpoena from Trump, and; b) allowed Trump to provide written answers without addressing obstruction.
The investigation could not reach a conclusion on obstruction because it could not decide whether Trump had a “corrupt” state of mind without interviewing him. At the same time, Mueller says that he believed he could have sought a subpoena and forced Trump to testify, but chose not to do so for reasons that did not seem compelling.
A second surprising part of the report: At the end, Mueller argues that the president does not have a constitutional defense to a charge of obstruction. Trump’s lawyers argued, I think to good effect, that Congress could not pass laws criminalizing a president’s exercising of his constitutional powers, such as the power to remove federal law enforcement officials.
I believe that this is the correct reading of the Constitution’s separation of powers—otherwise, Congress could make it a crime simply to fire an executive branch officer unless Congress approves. Mueller improperly seized the power, which is actually vested in the attorney general, to interpret the Constitution and the scope of the president’s constitutional authority.
The most important thing for Trump going forward, is whether the House will initiate an impeachment investigation. Mueller tacitly invites such an investigation in his discussion of why DOJ doesn’t indict presidents (so as to permit other constitutional processes to occur, i.e, impeachment), and why he went ahead and conducting an investigation on obstruction anyway (to preserve evidence and fresh testimony).
Also read John Yoo's op-ed in The Washington Post: It's now impeachment or bust. As it should be.