The French
may be applying a particular provision of the Montreal Convention. The French made particular note that there were French nationals on board.
Below is the United States claim to jurisdiction for any crime on a foreign-owned aircraft flying outside the airspace of the United States, where a United States national was on board.
In 1996, § 32(b) was expanded to encompass foreign aircraft where a national of the United States: (1) was on board the aircraft, or would have been on board the aircraft if it had taken off; or (2) was a perpetrator of the offense. This extraterritorial jurisdiction of 18 U.S.C. § 32(b) is comparable to that currently found in 49 U.S.C. 46502(b) (formerly 49 U.S.C.App. § 1472(n)) (aircraft piracy outside special aircraft jurisdiction of the United States), and 18 U.S.C. § 1116(c) (murder of internationally protected persons).
It would be an interesting legal excursion to examine whether an aircraft instantly on hitting the water, or ditching and floating for however brief a time, becomes a 'vessel', and thus subject to admiralty law. This would give extraterritorial jurisdiction for crimes on vessels on the high seas -- high seas being outside the waters of any nation-state -- when the crime involves a national of the nation-state claiming jurisdiction, or the property interest of an individual national or entity of the nation-state.