Liberalism in general is keen on justifying a positivist world view, that is to say, human beings have the power to change the world for the better. To that end, strange ideologies wind up in bed together. Maybe it's the Hegelian synthesis at work. But it's not only liberals who have sought common ground with Islam. Under Reagan, the Mujahideen (jihadists) of Afghanistan were supported as a bulwark against Soviet expansion. Early in the 50's Eisenhower sided with Nasser of Egypt against French and British interests in the Suez Crisis. Nasser was later instrumental in promoting the pan Islamic identity that we are accustomed to accept as the natural state of affairs today, though it is a 20th century phenomenon, and nevertheless, Islam remains divided along traditional lines--Sunni, Shia etc.
Historian and former Catholic nun Karen Armstrong wrote a soaring apology for Islam in her book by that title. It could hardly have been more accommodating to Muhammad, the Koran, his early followers, their many military conquests and their often brutal methods. The question remains, why is she so willing to gloss over the bad part of Muslim history? To be fair, Christians are biased to their own tradition, and chalk up the violent actions of their forefathers to being men of their time. We ought not to judge the past by today's standards, they often argue.
I think the answer is spiritual. Lies, though they may contradict each other, all serve to contradict the truth. Lies spread darkness; truth spreads light. That light is Jesus Christ, and those who are of the darkness hide from the light so that their deeds may not be revealed. (John 3:20)