Fighting a war to liberate Poland only to hand it over to one of its attackers was a betrayal.
The Wallies made it absolutly clear to the Poles before signing the alliance that it was directed against Germany only. I didn't mean that they hinted at it, or implied it, but literary said to the poles in the secret part that "attack by country" in the document text is to be read as "attack by germany". The Poles knew this, accepted it, and made the alliance.
So if I tell you I will protect you against Germany only, and absolutly nobody else, especially the Soviets, you hear this and say "ok" and I then proceed to protect you against Germany, but not the Soviets...how did I betray you?