Many, if not most, of the "foiled terrorist plots" were actually instigated by the FBI. They recruits the "terrorists," instigate the attacks, suggested targets, conspired on methods, provided materials and training, and talked the dupes through the entire process, then busts them.
How many would have done what they did if they had not been talked into it by the FBI?
Is Boston a case where it "got away from" the FBI?
Did the brothers catch on tonthract that they were being set up, then followed through on their own with real explosives?
One has to wonder, based on the afbi's history of instigating so many other "plots" so they could "foil them and save the day," bolstering their prestige and growing their operations and funding.