""Fabricating evidence in a legal case and proceeding.

This does occur and I think does raise a point from the Wikipedia article about the original trial:Unlike the juries in similar prosecutions against AIM leaders at the time, the Fargo jury were not allowed to hear about other cases in which the FBI had been rebuked for tampering with evidence and witnesses""

Law enforcement agencies have fabricated evidence in other cases in the past, (no surprise there) and likely will do so in the future. No conspiracy theory is needed to establish that. What is and is not allowed in a trial is complex, and I suspect is considered differently in different cases and by different judges. If something is not allowed that should have been would be grounds to be considered in a legal appeal. The appeals process considers if evidence should have been allowed, and if itshould have been, and it wasn't is likely to have changed the verdict outcome.

Additionally, IMO, it should not be a surprise that the murder of "on duty" law enforcement agents is considered more significant in the legal system than the murder of the average Joe.