The answer is also self-evident: anti-Israel fanatics are anti-Semites.
Take any issue and determine what a free, rational, Christian would think about it - the Left will always be on the other side. Abortion, homosexuality, business ownership, patriotism - you name it.
Since our society is based on Judeo-Christian ethos, they're opposed. Since Israel is based on Judeo ethos, they're opposed to her existance.
It's a perfect fit with the Islamofascist worldview. Funny how they never notice - NOW is now on the same side as the Taliban, the same side as patriarchial men who viciously oppress women. GLAAD is now on the same side as those who push walls onto homosexuals. Interesting, ain't it!