I am sure that there were many people who understood even back then (1938!) how amazingly horribly asinine his recipe was. Anyway, it was 1938, what is our excuse now? Why would anybody follow his advise when we know what happened?
To answer the question, the pic I posted above ... is from the Rachel Corrie Foundation website, which gives you some idea of what prompts his slobbery rant.
I think the motivation is two-fold. The smaller part is driven by antisemitism, and the larger part by some short-sighted and ill-considered opinions centered on "being mean to underdogs."
In the case of Israel, I seriously believe that liberals are so uncomfortable with the idea that the Israelis are devoted to agressive self-defense, and so married to the idea that they're being "mean" to the Palestinians, that they'll do anything to justify the horrors committed by the Palestinians.
I seriously doubt that they have consciously evil intent; they're just so monumentally stupid, naive, and ideologically constipated that the consequences of their actions are undeniably evil. I suppose that makes them even more dangerous....