* Juries cannot be punished or reprimanded for coming to the "wrong decision". * A defendant who is acquitted cannot be tried again in front of a jury because of the defense of double jeopardy.
The reason for that bias might be more complicated (the view of the local community which will be selected for a jury might be influenced through various means -- but that's a secondary question to the primary point that there is a statistical bias regardless of the reason for it).
The alternative was him being locked in self-imposed exile forever. By getting him out, extraditing him, and finally sentencing him to a min term sentence, they can say they did something and still let him go in a reasonable amount of time.
Why would USA do this?
Since when has any country acted with benevolence towards those individuals it deems its enemy?