Here is an interesting article on who IS is able to recruit. Most are not devout Muslims. They are disaffected youth brainwashed into joining.
So any policy which creates isolation or hopelessness of young male Muslims will likely increase IS ability to recruit.
http://bigstory.ap.org/9f94ff7f1e294118956b049a51548b33
So perhaps the Islam isn't the problem leftists are correct despite the underlying anti human rights views that the surveys reveal.