There are stones like this in Poland commemorating Jews, e.g. former German city of Breslau, nowadays Wrocław. I believe the resistance came from the fact that Poles were not organizers of those killings/deportations and it was expected to be misunderstood by people unfamiliar with history (e.g. stone commemorating murdered Jews in a city that was formerly German but nowadays Polish, or Jews murdered in Poland by Germans).
Important to mention: there were killings (not supported by the state) of Jews and systemic antisemitism (clearly supported by the state) in Poland conducted and organized by Polish people before and during the war (this was, though, not supported by the state at that time), but incomparable in scale.
If you care about this, the "article" is wikipedia, and in the time you wrote this posts you could have added it already.
edit: I was quick to comment, the article is protected. So unless you already have an active Wikipedia account, you'd have to request an edit on the talk page.
It's hard to tell if this is some anti-Polish resentiments of one or many of the authors, or just extremely low level of historical knowledge, but something tells me it's the first.
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.