In fact, the opposite is true, the burden of proof was in fact on the DOJ to prove the murder-for-hire scheme, which was, as you can see, charged, and they met that burden at trial.
https://reason.com/2018/07/25/ross-ulbrichts-murder-for-hire...
Why were all the actual murder for hire charges dropped and instead included in a side notes of a conspiracy sentencing? Seems obvious to me that it was because there is no way they would get a conviction.
> The sentence was based on judicial findings related to allegations of serious crimes that not only were never found by a jury but were not even among the charges leveled at trial. During closing argument, the U.S. attorney explicitly advised the jury: “[T]o be clear, the defendant has not been charged for these attempted murders here. You’re not required to make any findings about them. And the government does not contend that those murders actually occurred.”
[1] https://www.supremecourt.gov/DocketPDF/17/17-950/34432/20180...