His lawyer tried to argue that his offense was not illegal under UK law, among other arguments including that it was disproportionate to his crime to be extradited.

The ruling spends several pages discussing it, and the ruling concludes "Accordingly in my judgement I am satisfied the conduct alleged in the instant request meets the dual criminality test and would be an offence in this jurisdiction."

In other words, he only has to obey British law. The only reason the ruling was in favor of extradition was because the crime he committed is also a British law punishable by at least one year.

However, he hasn't been prosecuted in the UK. Why not?

There's a saying that, if your dog must be shot, it is best to do it yourself. Farming it out makes the act worse, not better.

If a UK citizen is suspected of committing a crime in the UK, (s)he must be tried in the UK under UK law and, if found guilty, punished in the UK. The suspect should never, ever be extradited to a foreign power and punished by them.

It isn't so trivial to say where the crime is committed when the internet is related. He is being prosecuted for facilitating copyright in the United States. The people who committed actual copyright infringement in this case were in the US when they committed their crime, and he was facilitating them remotely.

The issue is being argued the same as if someone was selling pirated CDs in the US, and he was facilitating those people and making money doing so in the UK. Regardless of how you feel about the nature of digital copyright law, it's really not quite as ridiculous as you are making it sound.

I disagree. He's British, and allegedly committed a crime in Britain, against British law. He should be tried and, if found guilty, punished in Britain. Why is that not happening?

It may be the case that he has also committed a crime in the US. If Britain feels that the crime he is supposed to have committed in the US is heinous enough in nature to extradite him, then the proper procedures should be followed of course. However, if the crime is not heinous enough to cause a prosecution of him in Britain, it is not heinous enough to extradite him to the US.

In Thailand, I believe it is illegal to insult the person of the King. If someone in the UK insults the Thai king on the internet in a racially-aggravated way (and funding the site via advertisements), but not heinously enough to cause a prosecution in the UK, should they be extradited to Thailand to face prosecution there? I'd argue that they shouldn't and that this fictitious case is almost identical in nature to the one under discussion.

Your allegory is flawed (I believe) as I cannot imagine how one could facilitate copyright violation for pirating CDs remotely, in a way that is merely speech and is illegal in the host country and would cause extradition to face prosecution. Can you fill that detail in for me?

(The idea that there is a crime of facilitating copyright violation is nauseating, by the way. That's similar to the UK's offence of possessing information that may be of use to a terrorist. In practice, almost anything can be outlawed that way; the law is so broad as to be useless for punishing any single offence and capable of punishing any offence.)

Sorry, the idea that people can be extradited for providing directions to websites that enable you to break copy-write, but not say creating a gun which is actually used to kill someone is ridiculous.