The police didn't have enough information to arrest them until recently. The terrorists were under surveillance. The terrorists, like most terrorists, waited until the very last stage to get their hands on the money, an event that had a lot to do with the arrests. They almost always wait until the last stage to get the money necessary because, as in this case, it makes them look very suspicious. If they had not been followed for a year and a half, there would be a lot more innocent dead people right now. Their plan involved blowing up a forty mile fuel line, and that is the one part the experts are saying the terrorists could not have accomplished. Their plan would have still resulted in a whole lot of innocent dead people.
The fact that they are not members of Al Qaeda is not evidence that they should not have been followed and arrested. We are dealing with terrorists groups other than just Al Qaeda, as evidenced by this situation.
Imagine having a plan to fly somewhere this weekend and having to have a layover at the JFK Airport. Do you want suspected terrorists there to be under surveillance, or do you want them to be ignored?