Guess someone didn't told these guys how Poland was attacked by the USSR in '39.
[Super Simplify]Operation Barbarossa kinda left the Soviet's in need of Allies. Then things like the Sikorski–Mayski agreement tried to re-establish diplomatic relations. Then I guess you could call the Russians and Polish 'reluctant coworkers' for most of the war.[/Super Simplify]
^this.
Btw, those guys who fought on the Soviet side are the very same people who were beaten and gullaged after the '39. I'm pretty sure they remember very well themselves how Poland was attacked by the USSR. Lucky for them, Hitler falling out with Stalin two years later gave them oppourtunity to leave Gulags and join freshly created Polish army under the Soviet Command. Since their first battle in 1943 they fought all the way untill the Battle of Berlin. They didn't complain. Politics didn't matter all that much for the regular people, they just wanted to fight back for their country again, Poland didn't exist at the time, with its lands completely occupated by the Germans who wanted to turn polish citizens into a cheap/slave labour for the German "master race". There was something to fight for.
Polish Army on West never had the opportunity to actually fight for Poland on a Polish soil, unlike these lads.