Possible Reason 1:
She was not guilty of anything. This could only be due to the fact that she did not reveal an Official Secret. The only way this situation could be true, is if the information she gave had been faked. The revelation of a lie, or a fake, cannot be considered to be ‘giving away a secret’. It could be considered to be ‘muddying the waters’ … but that is about all. A secret can either be ‘a secret’ - or not exist. (The +content+ of the secret +itself+ could, of course, be true or false e.g. "45 minute claims" … but either a secret exists … or it doesn’t. Either a table or a chair exists - or it doesn’t).

In this case, if there was no secret to give away, then she could not have given it away. Therefore she could not have breached the OSA. Therefore she wuz innocent, an’ walked … free as a bird, yer Honour.

But then, if there was no secret to give away, why was she sacked from GCHQ? Why was she summonsed, in the first place? How can giving away a completely non-existent piece of information put a person in the position of being summonsed under the OSA? Is ‘muddying the waters a little’ a part of the OSA? If so, which Section?

The only sensible answer is Possible Reason 2:
It’s all true. Absolutely true. She revealed a secret that - during a trial - would have undoubtedly revealed even more jealously guarded information. And therefore the only two questions that remain are:
(a) Whether or not this additional information would have embarrassed and already-seriously-embarrassed HMG, and
(b) Having been requested to spy on the UN Secretary General, did we actually do it?
And, as everyone appreciates, these are extremely serious questions and need to be answered. Now. (Not in 50 years time).

The only other possible answer to the original question is Possible Reason 3:
We are all either utterly stupid or stark, staring, bonkers.
It has often been said: In any war, the first casualty is always the truth.

I cannot see how any judge could rule that this was germane to the case. Making war or peace is a Crown prerogative, no more subject to Parliament in itself than it was in the days of Good Queen Anne.

Ms Gun accepted the terms of the Official Secrets Act when she took HMG’s money and has no more right to her employer’s correspondence than anyone else, except as it relates directly to her terms of employment.

The claimed "illegality" would presumably be a breach of the UN Charter, in which case any legal fisticuffs would be between the UN & HMG . As far as I know the UN has no power to make British domestic law, to which Ms Gun is subject.

Interestingly, a parliamentary committee ruled that the Kosovo war was "illegal" in terms of international law but also a "Good Thing" as per "1066 and All That". Bring on Gilbert & Sullivan!

When you work close to government, you get to sign a chitty just to draw your attention to the provisions of the Act; when you leave the work, you sign again to confirm that you have been reminded about the Act. That way, there’s no opportunity for claiming ignorance should you breach the Act, whether deliberately or inadvertently.

So the stuff about who signed what and when is largely irrelevant. Both Ms Short and Ms Gun are bound by the Act as law in perpetuity. Any decision to prosecute them under the Act is based only on a) whether they actually breached it in fact and b) whether there’s a net benefit in so doing.

State tradition is the last refuge of a scoundrel.

They say, Gun signed up to the OSA and so therefore has to eat it, but the UK signed up to the UN charter much earlier, and shouldn’t breach that.

If there was a committee, system, or organization that was empowered to censure the "security" services inasmuch as to judge the difference between spying for the purpose of protecting the country from war, and spying for the purpose of propelling the country into war, then breaking the OSA might be considered naughty.

But there is no body to which state crimes can be reported and dealt with in this country, so going public is the only alternative.