In essence, GCHQ & NSA have constructed a giant totalitarian state, with almost unlimited power. The only reassurance that it's not to be used against us is "trust us". However much we might trust them for now, that's simply not a good enough guarantee. Like Checkov's gun, the very existence of this weapon means that it almost certainly will be fired, eventually.
The only way for us to ensure our safety is to make sure that the weapon is put beyond use, ASAP.