This is an extension of everything the US government has been doing wrong for the past several years. Every time they get caught with their hand in the over reach cookie jar, they yell "it is a secret and it must remain so". The gag order is nothing more than an attempt to hide the truth. They harassed a person to the point that he got fed up, and he spoke out inappropriately. Then they went after him just to shut him up. Talk about thin skinned.
The issue here is, it would undermine not just this one case, but all the FBI "created terrorist" cases. It would show the FBI in a bad light, and they do not want another agency under the spotlight. NSA, IRS, etc ...
Cameras can not help you stop crime, they allow you to determine what occurred at a later time. You need actionable information before hand to stop any sort of event, and surveillance through cameras or the NSA's spying doesn't give you that, everyone knows its there. All it does is wastes resources and gives the illusion of safety.
Ask a mathematician about this, the more data you collect, the more false positives and false negatives you can expect. You end up flagging a ton of people who shouldn't be flagged, and investigating them and wasting resources. Plus you miss more of the people that are about to do no good. In the end this sort of surveillance makes us less safe.