I think they like to think they are standing up for the underdog. They like to rally around the groups they consider to be the downtrodden. I guess it makes them feel better about themselves.
The left has nothing but blind, unrelenting hatred for Western values and civilization, especially as practiced and represented by America. The Islamofascists share this hatred, so naturally the left allies with them.
Thing is, the left is as equally blinded by their own arrogance and hubris as they are by their hate for the West. They actually think that once the Islamofascists bring down the West, they will be able to sit down and reason with them to spare their own sick, perverted nihilistic selves, and their lifestyle and world view.
The left thinks that they can tame the crocodile, and in so doing he won't eat them last after all. But crocodiles can never be trained.