One should look at the terms the UK and France guaranteed regarding Poland. Their defence agreement had the specific Stipulation that they would come to Poland's defense in the event that its national Sovereignty were threatened. In other words, if Hitler did the same thing that they did to Czechoslovakia; invade and swallow it up into Nazi Germany. Eventually or course they did. Western Poland/Warthegau was annexed, and what was left became the General Government; a colony under German administration.

Even though they did invade Poland, the Soviet invasion was presented differently. Two decades ago, in the Soviet Polish war, the Poles seized areas of Belorussia and Ukraine after driving the Soviets back. Stalin's invasion was presented as them taking these areas back. I.e. it did not fall under the stipulation specified in the UK and France's guarantee.

Not sure why it's claimed no one condemned the Soviet Union. Plenty of people condemned them at the time it happened. For example, the UK supported Finland when the Soviets invaded them in the Winter War. Quite a lot of people in NATO today also remember that the Soviets also invaded Poland and are quick to remind the Modern Russian government about it when they deny it.