They have broken TOR and don't want others to know how they did it - worth more to them than letting a few kiddie porn sickos off the hook.
The good thing is that the case was thrown out (even though the perp needs to be put away)- if they can't prove their data is valid, they shouldn't be able to use it - it becomes hearsay....
This is a bit of conundrum for the FBI, if they aren’t willing to reveal how they have compromised TOR to charge criminals, then how do they propose bringing them to trial?
It’s almost self defeating.