I really don't understand why you have theorised that a US attorney would allow attacks to take place.
Obviously an attorney is seeking closed prosecutions, and the article is about closing prosecutorial cases against people "plotting" attacks, and, therefore, no additional steps are required in order to obtain prosecution.
It is possible that they don't always using their skills legally and under supervision. It is like computer security researchers - some are law abiding, some are a bit dodgy and do things that aren't legal. With FISA in the mix we can't even be completely sure what is legal in this area anyway.
[0] https://en.wikipedia.org/wiki/FBI%E2%80%93King_suicide_lette...
They have a tactic they could employ in this case, commonly called "lying". Remember how FBI lawyer lied to FISA court? How long was his prison sentence after he got caught, what do you think? Trick question, because he didn't get any prison time at all. He also haven't lost his law license, because lying to the court where there's no adversarial process and that wholly relies on honesty of the lawyers to suspend human rights of citizens - is not grounds for disbarment.
This is what happens if you get caught red-handed lying in the probably hottest case in the history of the nation. In smaller cases, there's usually not even an indictment, people just shrug and move on. Testilying is routine enough to have its own word. The practice is routine enough to have a long Wikipedia article [1] (actually, two articles). Practically never prosecuted. Of course, when and if and comes out, the US Attorney who was on the receiving end probably would be pissed. So what. If the goal worth it, it will be done. They'd just present it as a routine operation, lie in the sensitive parts, and nobody will know it until it'd be too late. Or maybe long after.