As my earlier post states, this is incorrect. It is generally true that the laws of any given country only apply to its citizens, and to non-citizens within its sovereign territory. However, many countries assert the right to prosecute people beyond those bounds for certain types of crimes. Espionage is especially high on the list of such crimes because (as wikiLeaks has so clearly demonstrated) an anti-espionage law that stops at the border is practically impotent. Usually, such crimes are tried in absentia because the prosecuting country has no way to get their hands on the defendant (e.g. Osama bin Laden), but sometimes countries will go to great lengths to seize a suspect, such as when U.S. Navy jets intercepted an airliner carrying Palestinian hijackers and forced it to land at NAS Sigonella.
International law can be very quirky (IANAL, but my work sometimes requires me to deal very directly with particular specific aspects of international law). Much of it comes down to precedent: if you can enforce a claim consistently for a long enough time, it becomes accepted and legitimate international law, which is the reason for the U.S. military's Freedom of Navigation (FON) operations. Conversely, if you allow others to regularly violate your national sovereignty in a particular manner without responding (even if only by formal diplomatic protest), then those particular rights may come to be accepted as no longer belonging to your nation.
A meta-observation: I've noted that posts saying, "he can't be prosecuted by the U.S." are generally getting many upvotes, while posts explaining how and why he could (not that he SHOULD, just that it's possible) are getting few or none. I can't know what other people are thinking, but it seems to me that people are voting based on what they would like to be true, rather than on what actually is true.