You have to put yourself in the police shoes.
You want to find out about this guy but you know that hes really pretty internet savy - so you ask the obvious question - "Who is the most likely to be able to listen into him and not be found out by him" - answer GCSB
But when asked to help the GCSB see that they actually shouldnt be doing that - so to give themselves the OK they say to the police "If you tell us that this guy is a foreign national then we can listen in". They also assume that due to secrecy that this sort of carry-on wont ever be disclosed.
So the police -( who have by now done so many stupid things - so whats one more) tell GCSB that hes a foreign national. GSCB is off the hook.
But they didnt count on a set of rather smart (and costly) lawyers who would want to check out every inconsistancy, nor a judge who seems to have a dislike for corner cutting and dodgy actions on behalf of law enforcers.