I have always wondered this since the invasion of Poland was basically a German-Soviet joint venture - and indeed the Soviet invasion had a large impact on the Polish campaign - despite popular depictions of Germany smashing Poland the Polish army could have probably held for a lot longer.

The only valid explanation I can come up with is that France and the UK could not afford to also declare war on the S.U. after having just gone to war on Germany. This however does not explain why the public conception has not changed in the many years since.