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.

This existed before the internet as well.