Above: Soviet Foreign Minister Molotov signs the Nazi-Soviet Non-aggression Pact while German Foreign Minister Von Ribbentrop and Soviet leader Stalin look on under a portrait of Lenin, August 23, 1939. News of the Pact stunned the world and paved the way for the beginning of World War Two with Hitler assured the Germans would not have to fight a war on two fronts.
http://www.historyplace.com/worldwar2/timeline/pact.htm ___________________________________________________
"...at the time of the Nazi-Soviet Pact, the Communists suddenly transformed the 'Anti-Nazi League' into the "Hollywood Peace Forum," calling for American neutrality and using the slogan 'Let's Skip the Next War.'..."
--Ronald Radosh, from his book, Red Star Over Hollywood: The Film Colony's Long Romance With the Left
http://www.capmag.com/article.asp?ID=4251
I think everyone knows that this pact existed. Stalin obviously initially didn't want to get involved in the war (hence a nonaggression pact, rather than a full agreement to be allies).
You can give the USSR credit for helping win the war without making a moral judgement against the people. Yes, the government was Communist. Yes, Stalin was a bad MF. This doesn't change the fact that millions of men gave their lives to defend their homeland and defeat Hitler. These people were not somehow subhuman because their government was Communist. These were normal people. Wives lost their husbands, children grew up without their fathers.
Today, the Russians celebrate this victory because it is important. These celebrate it because regardless of the government then or now, it was a victory of the people against fascism. It is quite possible to celebrate the victory without celebrating the Communism of the USSR, and quite alright to celebrate the accomplishments of the Red Army (and display their symbols). Recognizing the sacrifice of these great men by using the flag they fought and died under is not something negative, but rather necessary and proper.