Stalin, after the victory in WWII, did not decide to include new republics into the USSR. He decided to establish independent countries such as Hungary, Czechoslovakia, Romania – and Poland.
This message is part of Russian historical revisionism and part of a disinformation campaign portraying Russia as the peace-maker and liberator. See other examples of Russian historical revisionism concerning Poland here, here and here.
The very fact that Stalin did not turn Poland, Hungary, Romania, etc. into new USSR republics does not mean that he ''established independent states'' in Eastern Europe after WWII. In fact, these countries were kept under repressive communist regime and unable to make independent political, economic and military decisions.
The Sovietization of Poland and other countries of the so-called Eastern Bloc after the WWII was accompanied by terror, nationalisation of industry, expropriation of privately owned land and restrictions of civil and political freedoms. The Warsaw Treaty Organisation (the Warsaw Pact) was the Soviet Union’s “external empire”, where people faced daily strife, including shortages of food and housing. Read more about life in Poland under communist rule.