While it is not justifying it, but is that really a major distinction. The purpose of several previous non-aggression pacts was to sign off on Nazi territorial annexation (again, selectively remembered with appeasement) of saying "it is okay to take the Sudetenland" and "We wont use the Anschluss as a Causus Belli". The reason the Soviets took the eastern part of Poland was to push the border west and that they felt that they could justify this by the fact that it was part of the Russian Empire and Soviet Russia during the revolution and that it was lost to get peace in WW1. That does not justify it, just that any talk about the Soviets, rather than France and Britain, being more compromised and accommodating to the Nazis or that the Soviets were more pro-Nazi than the West is demonstrably wrong and rewritting history for cold war reasons.