So you had a number of idealogical recruits that didn't belong to any particular country, that had access via their purported citizenship in this country, and they pursued legal objectives (pilot training) with an ill will in their minds ...
Heck - that fits any number of people in this country on any given day.
It's the intent followed by the execution of that ill will that causes problems. Ill will in this case that evolved into a plan to plow a plane into the WTC towers ...