because many agree with ISIS and silently support them. That's the problem.
That is fricken scary!
http://www.theatlantic.com/features/archive/2015/02/what-isis-really-wants/384980/
This is from the referenced article:
"In fact, much of what the group does looks nonsensical except in light of a sincere, carefully considered commitment to returning civilization to a seventh-century legal environment, and ultimately to bringing about the apocalypse."
and...
"In September, Sheikh Abu Muhammad al-Adnani, the Islamic State’s chief spokesman, called on Muslims in Western countries such as France and Canada to find an infidel and “smash his head with a rock,” poison him, run him over with a car, or “destroy his crops.” To Western ears, the biblical-sounding punishments—the stoning and crop destruction—juxtaposed strangely with his more modern-sounding call to vehicular homicide. (As if to show that he could terrorize by imagery alone, Adnani also referred to Secretary of State John Kerry as an “uncircumcised geezer.”)"
and...
"The reality is that the Islamic State is Islamic. Very Islamic. Yes, it has attracted psychopaths and adventure seekers, drawn largely from the disaffected populations of the Middle East and Europe. But the religion preached by its most ardent followers derives from coherent and even learned interpretations of Islam."
Not exactly a small area they control: