The difference between using nazi imagery and using socialist imagery is simple. Socialism advocates freedom and equality which means that anyone using genocide and oppression to achieve it's purposes it's an enemy of socialism. Nazism on the other hand openly advocates for racial discrimination and genocide. So nazi leader committing genocide would just stay true to his ideology while a communist leader doing this would be betraying his ideology. Leaders and governments shouldn't be judged for what they say but for what they do. Hitler did exactly what the Nazi ideology advocates so he is indeed a Nazi. Stalin on the other hand was a liar. He ,of course, called himself a communist, all dictators like to associate himself with ideals like freedom and equality but if we look at his actions we can see that he didn't live by this ideals. Stalin actions don't make communism bad they simply make him not a communist (or a bad communist, at least).
Furthermore, the majority of socialist ideologies don't agree with the more extreme actions taking in USSR. Most of us at LSF for example, being anarchist, disagree with pretty much everything about Soviet Russia and similar regimes. And all these socialist ideologies including most anarchist accept the Hammer ans Sickle as a symbol of socialism. There are no other nazi ideologies, on the other hand, so using the swastika and other nazi symbolism can mean only one thin. No nazi can say "I'm a nazi but I 100% disagree with everything Nazi Germany was standing for" while plenty of socialists say that about the USSR.
So while neither LSF nor Int use the Hammer and Sickle it would be nothing bad in they did so. While anyone using the swastika would definitely be a bad thing.