They are not there by accident though but by design. Muslims believe that the sign of the end of the world involves 'something' (which I forget right now, was told by a Muslim guy) happening in Syria yadda yadda...so it seems that they are trying to fulfill the prophecy.
This is what concerns me at the moment. I've heard something similar, and I think ISIS are playing on the 'fact' that all this is the beginning of the end as predicted somewhere by someone (or in the Koran, I'm not sure).
Lots of young, disillusioned and highly excitable muslims are flocking to the area to join ISIS and it's in danger of very quickly becoming some kind of horrible self-fulfilling 'phrophesy'.
For all the ISIS losses in the recent battles with the Kurds, their numbers are still swelling (both inside and outside of their so called caliphate) and that's a major concern I think.