It's all very simple. A quick perusal of Marxist / socialist history reveals roiling antisemitism in Marx, a self-hating Jew, and then specifically anti-Zionist fervor among second-generation, seminal Marxists ranging from Otto Bauer to Karl Kautsky.
Their opposition to a Jewish homeland in Palestine had nothing whatsoever to do with "racism" or whatever lefty soundbyte you hear bandied about today. It was borne out of a callous attempt to harness certain national movements and suppress others to achieve Marxist geopolitical goals, most notably the toppling of the Russian monarchy. It was also, paradoxically, the socialist prescription for combating European antisemitism; theirs was simply the opposite of that of the Zionists.

Nascent Zionism was seen as reactionary because it would worsen Jewish otherness by impeding their assimilation, distract Jews from the more worthy goal of revolution, and promote capitalism. The latter assumption came from Marx's view that Jews were a crude race synonymous with the evils of profiteering. He admitted to a degree that Jews were victims in Europe, but felt that the emancipation of Jews from antisemitism was dependent on "the emancipation of society from Judaism" (see his essay, "On the Jewish Question").

In other words, remove capitalism from society, and its quintessence, the Jews in their unassimilated Jewish form, and you promote a brotherhood that encompasses everyone, including Jews. If you bear in mind the internationalism of Marxism, you see how this goes beyond Western Europe into Israel/Palestine.

The events leading up to World War I made it clear that Marx was woefully wrong about his esteem for internationalism over the seductive pull of nationalist fervor. Marxists had to relax their doctrine with respect to nationalism to address violently changing realities. Most, of course, came to embrace nationalism, the most tragic and extreme example being Hitler's fusion of socialism -- yes, socialism -- with rabid ethnic nationalism.

The uniquely landless status of European Jewry ran against Marx's territorial principle, in which one people necessarily inhabited one land. (Ironically, given the obsessions of our modern-day anarcho-socialists, Marx was decidedly insensitive in this way to the exigencies facing minorities.) Marxists didn't know how to process such a problematic diaspora people. I imagine their unique, antagonistic space within Marx's capitalist critique didn't help matters. Therefore, socialist opposition to Jewish national aspirations in Palestine continued unabated.

It came to pass that Marxists opposed national self-determination for one group only: the Jews. This continued throughout history until the present day, as Marxists and anarchists blended into Leninists, Stalinists, socialists of all stripes (agnostic, third-world, even Social Democrat), the academic lefties and radicals of the 60s, and finally into the diverse and inarticulate hipster rabble you see now on Indymedia sites carrying pro-Saddam posters.

So, when you see the brainwashed kids scurrying around the Mall or the tofu killing fields of San Francisco, with their Star of David / Swastika placards, clamoring for the deliverance of genocidal dictators, remember what actually underlies the choices they make.