Whilst I have no doubt that internal politics played a big part in this with the people in charge after a big headline, I really don't see your problem with this.

The defendant made his intent to murder clear from the start, prior to the FBI introducing him to the bomb, however the sting proved beyond reasonable doubt that the defendant wasn't merely a fantasist - he actually pulled the trigger on a device that, in his mind, would kill as many people as possible. Don't you think that had the FBI presented an affidavit based on voice recordings alone, the defence would've torn them apart in court?

Whenever I read stuff like this, I'm thankful that 90% of homegrown terrorists are fucking idiots who fall for stupid stings like this. I'm pretty sure that at least 50% of the commune, if they wanted, could manage a terrorist attack.