Because historically the Polish-Lithuanian state was partitioned between Russia, Prussia, and Austria in the late 18th century. The core of Poland was, for all intents and purposes, Russian territory all the way up until World War I when it fell to the Germans. The poles spent over a century under Russian occupation.

Poland regained independence at the end of WWI. However, the newly created Soviet Union invaded it in 1921 but was repelled. By 1938, it has only being less than 25 years since Poland was Russian ruled and less than 20 years since it was last invaded by a Russian state. Given this context, it is easy to believe that any Soviet forces stationed in the country would very quickly turn into an occupation force, or at least as likely to be a threat as help.