well, because the resolution of Nurnberg tribunal will always conatain this. its a historical document. but there are newer papers that state the exact opposite.
Because allies and communists wanted to blame Germans for everything, Allies knew that Communists did it, but it wouldn't look good if Allies burdened that to Communists because they were "allies" during war and wanted to keep peace between each other after war, so they take opportunity that Germans did many bad things and they also added this.