The notion that the law of a nation applies to persons who are not on the territory of the nation and are not citizens of the nation in question is obnoxious to the concept of national sovereignty. Again, if extraterritorial application of laws to non-citizens is acceptable, the PRC can seek to arrest and/or extradite US citizens who have never been to China for actions taken outside of China. If on the basis of treaties, Australia want to prosecute Assange for mishandling American classified information in violation of Australian law implementing such treaties, there would be nothing wrong with that. I believe we have such treaties with some countries (I think the UK, possibly all of the 5-Eyes countries now), but the US has no right to apply our laws to non-Americans outside of the US, and the assertion of such a right created the right for other countries to apply their laws to Americans in America.
Says who? Your point about extradition is a red herring: the sovereign from whom extradition is sought has the sole discretion about whether to extradite or not, so the PRC couldn't do diddly or squat if the U.S. chose not to extradite.
The U.S. has every right to enact laws that apply to non-U.S. citizens for conduct that occurred outside the U.S. Under traditional international law there only needs some modicum of connection - aka nexus - with the U.S. in a substantive sense, and U.S. classified information is more than sufficient nexus.
From a Proskauer summary on international jurisdiction and the presumption against extraterritoriality:
Exception for Criminal Statutes: There is no presumption against extraterritoriality when dealing with statutes prohibiting crimes against the U.S. government. U.S. v. Bowman, 260 U.S. 94 (1922); but see Kollias v. D & G Marine Maintenance, 29 F.3d 67, 71 (2d Cir. 1994), cert. denied, 513 U.S. 1146 (1995) (holding that Bowman should be read narrowly, such that “only criminal statutes, and perhaps only those relating to the government’s power to prosecute wrongs committed against it, are exempt from the presumption [against extraterritoriality]”). Criminal statutes are deemed not to be dependent upon the locality of their government’s jurisdiction, but on the right of the government to defend itself against obstruction and fraud committed by its own “citizens, officers or agents.” Bowman, 260 U.S. at 98.
Notice, in particular, that the presumption (which can be rebutted) against extraterritoriality does not apply to criminal statutes or statutes relating to the government's power to prosecute wrongs against it. The abuse of U.S. classified information is a core wrong against the U.S. government qua government, and is therefore not subject to the presumption against extraterritoriality.
The U.S. has the right to make the abuse of U.S. classified information a crime by whomever committed, wherever committed. The question comes down to power - the ability as a practical matter to enforce that law - and not to the right to enact the law in the first place.
Link to Proskauer piece:
http://www.proskauerguide.com/law_topics/25/II
