This is not a good understanding of Extradition. Extradition before the age of the internet involved identifying people who had in the past committed a crime in a certain jurisdiction and transporting them back there to face justice. So for example, I murder someone, I flee to the UK, the USA asks the UK to extradite me back. Or indeed JA (allegedly) rapes someone, flees to the UK, Sweden asks for extradition.
What's at question here is: Given Assange was never in the US, how has he committed a crime in the US and the answer is that the US has a funny idea jurisdiction.
Crimes committed against the United States are seen as crimes committed in the United States. This is an issue of sovereignty, as there is no (for most intents and purposes) higher, international court to pursue the case in. Therefore, the case is tried in the United States. It's similar to any other foreign national conspiring in crime in the United States. They don't have to physically be in a country to break a country's laws.
The country that the individual was in when the crime was committed can certainly handle the issue and not extradite, however, generally these agreements avoid that.
If US acknowledged international courts, you could make that argument. But as of now, the US doesn't acknowledge some of the most important international courts, like the ICC. Thus the argument that there are no higher courts is entirely bogus, as it is so by choice.
That's the point of extradition agreements. If we're worried that Brazil's Senate has passed morally incompatible laws and wants to extradite folks from the US, then we won't agree. The State Department is (or tries to be) on top of any foreign legislation that would change our agreements.
I'm not talking about morality but legitimacy, why are you bound to follow laws that were not voted for representatives you chose on a country you never been?
That's why I think there must be limits regarding what reasons a country can use to ask for an extradition, specially if you want to extradite a person that's not even an American for treason, espionage or stuff like that.
The idea of extraterritorial jurisdiction has existed for centuries with piracy laws being some of the earliest examples. Most countries have enshrined the idea into their laws in some manner especially when it concerns laws in which the victim is the nation itself. Many countries go way beyond what the US has. For instance France claims the right to prosecute anyone for any crime committed against a French national regardless of where the crime was committed.
What's at question here is: Given Assange was never in the US, how has he committed a crime in the US and the answer is that the US has a funny idea jurisdiction.