I believe the case here is that the methods don’t produce reliable evidence, which is almost certainly the reason they don’t want to open those methods for inspection.
By hiding the method they can make any person they want to point to guilty, just by having the program say so. Or saying the program said so.
If the spy methods are not actually reliable, they should not be used, goes without saying.
But like with terrorism, we don’t have to inform the rest of the terrorists what we are privy to.