I'll concede my claim requires a belief in an absolute morality, which I don't necessarily, but take it my claim in the following form:

1. You believe that copyright infringement is a "wrong" committed by one party on another party (the victim).

2. A British national commits copyright infringement where the victim is in the United States.

3. The United States conceivably has a moral right to seek punishment against that British national.

I mean, to say otherwise would be to say that copyright existed within a border. We have treaties explicitly so that isn't so - if there were a place in the world where copyright law didn't apply, you couldn't post things on the Internet and expect it to retain its copyright.

I don't support extradition in this case. Don't get me wrong. I don't believe in an absolute morality, and I'm not even sure I believe that intellectual property is a good idea on balance, but the moral argument can be made if you're willing to speak in more concrete terms (i.e., if you're willing to accept the premise that copyright infringement is a global "wrong")