In this case the stones mark the places where the victims lived.
I assume most Polish victims did not live in the territory of present day Germany.
From personal experience I recall some with "eastern" names in a town that used to have a notable polish minority but I couldn't say for sure.
Btw Can you recommend places to visit to learn more about their stories and history (besides the former sites of the concentration camps)?
Think about where we would be if we hadn't gone down this path. We can all help move things forward if we meet each other with an open mind, and steer clear of the resentment and national chauvinism of the past.
The blame was always squarely put on Germany. It was always stressed that most of the German population had more than an inkling of what was going on and many agreed.
The first time I heard the term "polish extermination camps" was a few years ago when the Polish Government outlawed it.
So I'm really at a loss here.
The only sort of blame shifting related to Poland that I encountered in East German propaganda was that the Katyn massacre was blamed on Nazi Germany, not on the Soviet Union.
Of course there are extremely far right organizations in Germany which to this day try to deflect the blame, but they are a minority and don't influence the public discourse (unless you live in their bubble of fake news and propaganda of course).
But in general I don't see why nationality matters much in this case. When talking about persecuted Jews, communists, homosexuals (or any other group persecuted by the nazis) of course this also includes Polish Jews, communists and homosexuals.