They're actually happy and proud to find someone who has the same ancestry as them and they feel a kinship. For some reason, people from Poland have a problem with it. When someone's parents or grandparents were from Poland and they want to recognize that, it's not acceptable. Why is that?
It's because the American Polonia are idiotic to the point of hysteria about it. Look at for instance, the British Polonia from post-WW2. That generation has more or less successfully integrated - and while there's plenty of Polish names about, the grandchildren consider themselves to be British, not Polish. They accept that they come from Poland originally, but they don't go on constantly about it - they are British to all practical extents. Really, I can't say a bad word about the post-WW2 British Polonia.
Then you get the American Polonia, who are frankly idiots. They consider themselves to be Polish, yet they don't speak the language, nor do they have any interest in the country. They vote for people like Jaroslaw Kaczynski based on what's reported in the English press - and they constantly preach about how great it is to be Polish - yet they're about as Polish as I am - ie, not at all. The worst thing is that they have a very ...hmm, sentimental view of what Poland should be - and they get upset when they visit places like Krakow and discover that it's no different to any other place. Many of them are longing for Poland to be just how it was in "Busha's time". The fact they even use words like "Busha" tells you about them!
Then there's the fact that many of them can't even pronounce their name properly - I'm sorry, but you can't claim to be Polish if you can't even say your last name properly. Then you get the fact that many of them claim that their great-grandparents came from Poland - yet when you listen to them, you discover that they emigrated before 1920. Most of them have absolutely no concept that Poland didn't exist for 120 years or so.
But then again, what can you expect from the descendants of very poor Poles from the turn of the 20th century?