Well, it depends, if you are from one country and break the law while in another, you absolutely are subject to that countries laws, if you leave that country before they prosecute you are subject to extradition. Their argument is he was performing an intrusion on a server on US soil and is therefore subject to extradition.
One dicey bit here is if a digital presence constitutes a physical one, and that's probably determined by the extradition treaty.
Ultimately the dumbass probably would have gotten away had he dealt in Sweeden swiftly, getting the issue resolved before the extradition could go through instead of running away and hiding, essentially volunteering for house arrest without resolving this shit.
Word of the Day: Guillotine: the inevitable outcome of the neo-feudalist stage of capitalism.
"The medium is the massage." -- Crazy Nigel