Operating under the assumption that the Jews are a "race" (because there is more genetic evidence for the existence of a Jewish "race" than there is for a black "race"), I do not see how anyone could argue that Zionism is not racism. Zionism promotes the idea of a homeland for one specific race, ruled by one specific race and advancing the goals of one specific race. In most of the civilized world, phrases like "Britain for the Britons" are laughed at, but in Israel "Israel for the Jews" is a fundamental policy.
This, of course, speaks only to the theoretical implications of Zionism. In practice, it has displaced the Arab population of the land formerly known as Palestine, and even Arab Israelis are heavily discriminated against.
It is clear that, in theory and in practice, Zionism is a racist policy. So why do Jews get so upset when someone says Zionism is racism?