Once you agree with the Nazis on how to split the country, it makes sense to grab the part you wanted. But diplomatic cables reveal that the Germans would not have kept this part of Poland as they intended to uphold the pact with the Soviets. That's why there were extensive discussions to release these parts as independent states, so they could be controlled by the USSR as they had agreed.
Whether you agree on that the Soviets should have signed the pact in the first place is another matter, as it was a carte blanche to invade Poland gifted to the Nazis.