While he was not a US citizen, he is involved in a crime committed in the US. Just like if an American had hired an art thief to steal a painting from a Europea

While he was not a US citizen, he is involved in a crime committed in the US. Just like if an American had hired an art thief to steal a painting from a European gallery. The legal concept of extraterritorial jurisdiction is extremely complex and nuanced, but primarily falls upon the diplomatic power of the country that believes you broke their laws. (If you find this interesting you might enjoy reading about US v. Van Der End, in which a Dutch smuggler was arrested and charged in the US for sailing drugs to Canada) However all of this is moot because he made the mistake of traveling to the UK and is currently being held by the British. In addition to leaking US secrets he also leaked GCHQ secrets and can be charged under the Official Secrets Act if extradition fails.