Celebrating the Soviet Union is a no no. Stalin was a bad dude.
No one is explicitly celebrating the Soviet Union. They are celebrating the victory over Hitler and Fascism, it just happens that the Soviet Union existed at that time and the victory was under that flag.
Russians at least celebrate their history unlike the West that wants to cancel everything that ever happened. History is history, the good and the bad.
Stalin murdered roughly 3 times as many innocent people as did Hitler. Before Hitler turned on Stalin the 2 had signed a non-aggression pact, which basically paved the way for Hitler and the Nazi movement.