Liberals seem almost pathologically accommodating of that most illiberal of all the world's religions, Islam. My theory is "the enemy of my enemy is my friend" - that is, since the people most alarmed by Islamic extremism are conservatives, their hate for conservatives trumps what should be a natural disdain for a religion that sanctions, for example, barbarism toward women.
Additionally, liberals with a national audience will mock and slander Christians and Christianity in ways they'd never dare with Islam. May have something to do with the fact that Christians don't riot, behead people and blow people up when they're offended. Real brave, those liberals.