Also, it gave all the Commie's and fellow travelers in the press and Hollywood a way to do a "Hand Waveum" to avoid any publicity of Stalin's crimes. "Look over there, see what those 'right-wing' Nazi's did to the Jews !!!!"
Which gave them a two-fer. It also allowed them to paint the National Socialists (remember, a hated enemy of the Commie's/Stalinist/Soviet lovers) as "right wing" to avoid the "left wing=industrial scale murder" truth.
Misconceptions that continue to this day.