Difference between Polish and Soviet is not tiny pidgeon hole difference. It is giving proper attribution. Not everything was done by Soviet. And for that matter, Soviet does not mean Russian automatically either.

And specifically, Poland was occupied by Soviet long enough to deserve not tonhave own art attributes to them.

Also, literature before revolutions and after them is much different. Topics, style of writing, who could get published or become famous are all much different. You can't attribute to Soviet what was done after Soviet broke up.