The FBI doesn't like the official definition because they are a terrorist organization. There's a reason we joke every time the FBI "foils a terrorist plot" that they were the ones who instigated it in the first place. After 9/11, they'd find the dumbest kid in the mosque and feed him propaganda and supplies. Usually they would "heroically save the day!" with much fanfare. Usually. They also instigated the Governor Whitmer kidnapping. And that's just what got publicized.