If they aren't causing trouble on our land, they aren't our problem.
Yeah, but the trouble does come when they turn their attention to the western world.
And with the considerably large number of western muslims who travel to join IS and similar groups, they pose a risk as they're national of western countries and can return, with the training in terrorism.
And there will definitely become reason for members of IS to focus on the western world, whether it's a general hatred on the grounds of culture/religion, or it's because of our interaction with other muslim countries, or our political interactions with IS.
And if IS continues to grow then we'll quickly start trying to put in sanctions on any state that they develop.