This question came up about two weeks ago and /u/Balnibarbian gave the best succinct answer to this issue. The long and the short of it is that the treaty with Poland only pertained to German aggression. The timing of the Soviet invasion threw Allied foreign policy into disarray with some in the French and British government and public sphere calling for anti-Soviet actions up to a declaration of war, and others adapting a wait and see approach hoping for the German-Soviet rapprochement to collapse. The Norway debacle and the Fall of France rendered this debate moot since Britain would have to prioritize using its armed forces against Germany and her Italian allies.