Example if I were to hack a foreign gov't or company's computers while residing in the US and being on US soil at the time, I would still be breaking that country's laws and could be arrested and extradited to that country.
This is an agreement many countries have with one another. Assange broke US law and therefore could be tried in a US court.
Actually the order is the opposite: he received intel which had already been stolen - which is legal, see New York Times Co. v. United States - and then encouraged Manning to search for any more stuff, but never received anything more.
So, he did in fact encourage the theft of classified info?! Doesnt matter if it didnt produce any results. There was still intent. Secondly, do you think Manning is the first he tried that with? Don't be naive.
If so, then you must admit that America is imposing it's own (subjective and ever-changing, depending on the parties in power) view on the international theatre and that's where the concern starts to get exacerbated.