Sociologists and Anthropologists (my duel majors in college) attempt to study cultures and societies objectively, dispassionately, scientifically. It is relatively easy to do when studying ancient societies or remote and isolated cultures.
Some societies, cultures and religions work and work well for centuries. And by working I mean they thrive, gathering wealth, power, population size and geographic size. They have the ability to affect other surrounding cultures and spread their values, laws and morals.
If a culture or society works and thrives, that doesn't mean it moral, good, fair or free. It does mean the members of the society generally follow their rules and laws. Very very often, historically, these laws and rules are religious in nature and survive nations and governments.
Morals are passed down from parents to children, values that must shared by all and must be strictly upheld. For example marriage is discouraged outside of many cultures, ethnic groups and religions. This insures children receive the values, attitudes and strict moral codes of the culture.
ISIS is attempting to form a Caliphate in Iraq and Syria (and beyond). But the Muslim culture and the Islamic religion that is its base, including and especially Sharia Law, already have proven to be the basis of a culture and societal structure that works. Sharia Law and the overreaching Muslim culture have already survived hundreds of years through multiple nations and governments.
That does not mean it's good or moral. That doesn't mean its actions are fair or free. Most of us are horrified by the stoning of non-believers, the murder of the victims of rape or incest or the enslavement of people from outside the tightly closed society. We in the west, see these actions as savage.
Meanwhile hundreds of thousands of people from all around the world are joining ISIS. It's a virtual world wide mass migration. Here in the west we're trying to stop people from moving the Iraq and joining the armies of ISIS. Yet people do make their way to a society they see as moral, just and, especially, functional. They want the rules, the laws, the morals and the leadership to guide their lives.
We need to understand how cultures, societies, nations and religions all work to help us understand why other cultures often breakdown. We need to understand why some cultures thrive and their members prosper, while others live in poverty, riddled with crime and corruption.
We need to understand this in our own American Society and culture where, through our inaction, ignorance and neglect, seem to have said "In spite of our Tweets to the contrary, Black lives actually don't seem to matter." How is it that we allow this American Genocide continues unabated, ignored or tolerated by most of society. We need to understand why violence and crime are rampant throughout American inner cities and now are spreading into the supposedly safe suburbs.
This Wall Street Journal article by Heather Mac Donald is an excellent place to start.