Or... the system catches things like this all the time, this is just one that slipped through, and if the system wasn't in place, there would be many, many more incidents of this type.

Maybe. (This is normally "bear-repellent amulet" type thinking, but I think there's a specific exemption in this case, in that there are good reasons why you'd see no evidence of the system's effectiveness: the government doesn't like to get the public riled up by terrorist threats--terrorism is entirely about riling-up, after all--and so, when the terrorism is caught ahead of time, you'd imagine they'd sweep it under the rug. I might be wrong, though; this still might be a bad justification!)