I could claim that that's simply my interpretation of Buddhism. As it's my faith I am free to believe what I want; I alone dictate my personal religious beliefs.
The point I'm getting at is that irrespective of how grossly wrong or misguided a person's interpretation of a faith is, they can still claim that they subscribe to it, even if every other member of that religion actively opposes them. Thus, members of IS can call themselves Islamic all they want, and it may be what they believe, but the vast majority of Muslims worldwide hate what they're doing, and consider it to be unislamic. Thus, referring to them as Muslim, whilst being what they want, isn't really accurate at all (as in my pentagram-drawing, heart-harvesting Buddhist example).