ping!
There’s really nothing to stop them from returning.
What’s especially worrisome to me are those Americans who might have converted to Islam a few years ago, then travelled to the Middle East for training before this issue was quite as much on the radar. They could have expatriated to a socialist-leaning country in Western Europe, and incorporated into radical Muslim communities where nobody watches them. They’re free to return to the US for family visits any time without being put on a watch list.