A problem with secular shoehorning of grievances is its application in cases where it does not apply. This holds especially apt when dealing with religious motivations involving religions that extent from the mere intrapersonal to the interpersonal (e.g. the organization of society as a whole along religious strictures). Could one engage in insurgency from an ideology of secular deprivation? Sure. Could one engage in insurgency from an ideology of religious commandment? Sure. Understanding 'why they fight' requires understanding their motivations, from their perspective, rather than shoehorning their motivations into what one might wish they were.

As an aside, the whole concept of violence as a method for implementing political change as being 'radical', 'extremist', etc. smacks of epistemological bias.