One of the common refrains we hear in the reporting on ISIL is that officials are worried that Americans will go to Syria or Iraq, fight with ISIL, and then return here to launch terrorist attacks on the United States. As a discrete category of terrorist threat, this is something very odd to be afraid of.
It isn't that such people might not have the motivation to carry out a terrorist attack. But if they went to fight with ISIL, they probably already had the motivation. Ah, but what about the things they learned there? This morning, I heard a reporter on NPR refer to such returnees employing their "newfound terrorist skills" against the United States. But what skills are we talking about? If you want to learn how to make a bomb, you don't have to go to Syria to acquire the knowledge. There's this thing called "the internet" where it can be found much easier.
The way these potential attackers are talked about, you might think that launching a terrorist attack is something you can only achieve after years of intensive training in an arcane discipline, the secrets of which are closely held by wise old masters who deign to impart them only to carefully chosen initiates. But there's no terrorist Shaolin temple. If all you want to do is kill some Americans and sow chaos, it's actually not that hard. A couple of knuckleheads like the Tsarnaev brothers could do it.
This is an entirely separate question from whether ISIL as an organization wants to carry out an attack within the United States, because if they do, they don't need someone with an American passport to execute it. Anyone here on a tourist visa could do it (in 2013, just under 70 million international visitors came to the U.S.).
The point is that there are any number of reasons a person might decide to launch a terrorist attack against the United States. While it's entirely possible that an American could fight with ISIL and then get sent back to plant a bomb somewhere, that eventuality is no more likely than an American who has never left his home town becoming angry over U.S. foreign policy and deciding to lash out in the same manner. There are no "terrorist skills" you can't get here at home. So as a matter of policy, the returning expat terrorist is pretty far down the list of things we need to worry about.