Edit: According to the video link provided by spdy, the crime needs to exist in both systems (ie Hong Kong and the US) in order for an extradition request to be considered valid.
Treason, with a similar burden of proof, does not exist in Hong Kong, and thus could not be charged.
Is the US attorney's office allowed to rescind the charges of espionage and re-charge him with treason after he's been extradited to the US?