His "victim" was in the US and he knew that, so I don't think its so simple.
The problem I, and I think most people, have is twofold :
1 - There was and is no clear legal framework for intro-jurisdictional crimes like this. The law should be settled and clear before it is applied, otherwise people cannot asses whether their actions will break a law.
2 - The US criminal charges and punishments with which he is threatened appear to be grossly disproportionate to his actual actions. This wouldn't be an issue if he were guaranteed a fair trial, but the connection between the "victim" and the "prosecutor" (ie. both US gov) and statements made by the prosecution and US Government suggest that he may not get a fair trial.
The location of his victim is irrelevant IMHO. He was using a computer in the UK. Under the Computer Misuse Act, one must not use a computer to access another computer without prior authorisation of the owner, which he did not have. Therefore, he has committed a crime under said act in the UK.
I feel quite strongly about this, because if I were to ever commit such a crime for whatever reason, I would want to be prosecuted in the UK. Therefore, I should expect the UK to prosecute people in a similar situation.
There was a number of UK online gambling website owners arrested in the US for "selling" their services to US citizens. They were providing a service to UK citizens on UK (or at least non-US) servers and, at the time, didn't prevent people from signing up based on their IP (or other geographical information). The US felt it was their right to step in...
* I am not making an assertions on anyone's guilt, since they've never actually been proved in court.
The proper place to be prosecuted is the one in which the wider interests of justice and society are served. To that end the location of the victim and/or the perpetrator are both irrelevant.
It is a very nuanced question and perhaps varies a lot depending on the (alleged) crime. Garry's case is different to the gambling one you cite, he specifically chose and targeted a "victim" he knew to be in the US, the gambling websites didn't.
A parallel case may be where scammers in one country where the law doesn't prohibit it specifically target victims in another, remotely. In that case there would be no re-dress for the victims and justice and society would not benefit from your jurisdictional preference.
Until all countries have equal and fair laws, or there are definite and settled frameworks for jurisdiction, the cases will have to be judged on a case by case basis. Which is no good and leads to travesties like Garry's case. If you read my other comments, I am in favour of todays decision. But it's not a cut and dried case either way.
The problem I, and I think most people, have is twofold : 1 - There was and is no clear legal framework for intro-jurisdictional crimes like this. The law should be settled and clear before it is applied, otherwise people cannot asses whether their actions will break a law. 2 - The US criminal charges and punishments with which he is threatened appear to be grossly disproportionate to his actual actions. This wouldn't be an issue if he were guaranteed a fair trial, but the connection between the "victim" and the "prosecutor" (ie. both US gov) and statements made by the prosecution and US Government suggest that he may not get a fair trial.