The US has proscecutorial jurisdiction on crimes committee on civil aircraft owned/operated by American companies. (It also has jurisdiction over crimes committed on foreign aircrafts on flights that land the US, and that's so that US officials can legally board the plane and arrest the person once it lands.)

This and related laws were put into place after a pilot was attacked in 1950 while flying over international waters. Even though it was an American civil aircraft both departing from and landing on US soil, the US had no laws on the books giving it jurisdiction.

Because the crime was committed over UK soil, the UK also has jurisdiction and they prospected his co-conspirator back in the day, but are letting the US take this one.