Seriously? They will have no shortage of poor desperate stupid people to fight and die for an insane cause, but how many computer scientists, who could work anywhere in the world, want to go work for this insane organization?
A lot. A simple google search for "engineer terrorist" without the quotes turns up plenty of references if you want to learn more.
A terrorist is different than ISIS. They're not the same thing, and I just don't buy that a lot of intelligent people are joining ISIS, although there could be good ideological reasons to be a terrorist (none that I agree with).