"It was always an uphill struggle for Mueller, since collusion itself is not a crime. In other words, even if he could show that individuals in the Trump campaign had colluded with Russian agents to help elect Trump, that would be a serious political sin, but not a federal crime. Even if Mueller could prove that members of the Trump team had colluded with Julian Assange to use material that Assange had unlawfully obtained, that, too, would not be a crime. What would be a crime is something that no one claims happened: namely, that members of the Trump campaign told Assange to hack the Democratic National Committee before Assange did so. Merely using the product of an already committed theft of information is not a crime. If you don't believe me, ask the New York Times, the Washington Post, the Guardian and other newspapers that used material illegally obtained by Assange with full knowledge that it was illegally obtained. Not only did they use information from Assange, but also from Chelsea Manning and from the stolen Pentagon Papers. The First Amendment protects publication by the media of stolen information. It also protects use of such information by a political campaign, since political campaigns are also covered by the First Amendment."
The article:
https://www.gatestoneinstitute.org/13363/robert-mueller-modus-operandi