One reader response:
Why bother coming here (getting a visa, the travel, feeling out-of-place) to attack Americans, when one frustrated youth with jihadist tendencies from wherever can simply wander into Iraq or Afghanistan and do his best to take out US military personnel in uniform? There, you can at least blend into the local population, target not unarmed US civilians in shopping malls, -but uniformed, patriotic, heavily-armed Americans (who are light-years away from home and family, and all that's familiar) - and be regarded as a hero to millions ... all at the same time.
The flypaper theory works, in other words.