If Person A steals from (or rapes, murders etc.) entity B, then the nullification of their crime by some kind of international legal quagmire would be a miscarriage of justice.
The question is whether or not Assange materially participated in the theft of classified information from the US government.
I'm sure if US citizens stole from the German government or private entity, they would respect the legitimacy of the German governments concerns.
This is why we have at least some international laws and legal conventions and this issue should see it's day in court, after which, we'll have to see how well the proceedings went, and what kind of evidence is brought to light.