One, the simple fact that many Poles harbored and still do at least some anti-Semitic sentiments (as many, maybe even most, Europeans did, and and still do today). It's difficult for us to recall that sentiments and expressions we now consider outrageously anti-Semitic were considered normal in the West (even in the United States) before WWII. The war, I think, changed a lot in Western Europe (although they seem to be reverting to the historical norm now).
However, in the case of Poland, whatever amount of antisemitism existed was never generally understood any in historical period with which I am familiar as a license to kill or attack Jews, and there was no tradition of pogroms in pre-partition Poland.
Second, a large number of American Jews are descended from unassimilated, Yiddish speaking Jews fleeing Russian sponsored pogroms in the 19th century. My guess is that to many of the Jews, having lived for centuries in what had been parts of the Polish-Lithuanian Commonwealth, every Gentile was a "Pole". This folk misidentification has simply been transferred from one generation of American Jews to another. Anti-Polish propaganda (supported at least in part by Zionist organizations between the wars) didn't help either: I recall reading in a 1920's or early 1930's Jewish newspaper from Canada about a "pogrom" in Cracow. If I recall correctly, the death toll was 3 Jews and 4 Poles: not exactly a successful "pogrom". It was clear that they were talking about an ethnic riot, and not some organized or state-sponsored attempt to kill Jews or to force them out. The article was simply a propaganda piece to support Jewish immigration to Palestine.
Exactly