As to the reasons for this phenomenon, I would say there are many, not the least of which involves the following.
1 -- an oppositional defiant disorder based upon lack of self worth. These numb nuts don't know what they stand for, only what they are against, and their self hatred leads them to identify with Islamists in their nihilistic need to destroy their own culture.
2 -- utter conformism. They are the left's equivalent of the extreme religious fundies of the right, as they don't ever ask WHY they should believe in something, only that they are expected to do so. It acts as a conditioned response where unintelligent people react to positive reinforcement from their peers if they support Islam and negative if they don't.
3 -- post colonial guilt. Several decades worth of hammering home themes that all the world's ills are the result of the west has led to a mind set where the very reversal of the old cultural chauvinism now holds sway. People have lost the ability to see any merits whatsoever to their own liberal western cultures and have elevated the primitivism of other cultures to be beyond reproach.
4 -- fear. In a strange sort of projection, those who feed the crocodile hoping it will eat hem last call anybody who doesn't want that to happen the fearful ones. These people are actually so fearful that they refuse to learn anything about Islam while they live in their world of platitudes and ignorant group think. In an Orwellian expression of the maxim that ignorance is strength, anybody who DOES know a thing or two about Islam is automatically assaulted as being "racist", "Islamophobic", "Bigoted" or........, gasp! -- the ultimate bogey man -- a "right winger" It doesn't make any difference to these extraordinarily stupid people that it is actually Islam that represents the most extreme conservatism imaginable as they haven't the foggiest notion whet the terms even mean. They are just so frightened of being called the very names they call others that they lend themselves to the very agenda that represents the very worst bigotry on the face of the planet.
I swear, it's like watching some sort of group psychosis in action it is so absolutely moronic.