It looks like some (many? all?) of these flights were scheduled to stop, refuel and continue on. Perhaps the plan was to smuggle weapons on board through the lax security of Paris—remember, French police escorted Richard Reid and his plastique-packed sneakers onto that plane—wait for the planes to be refueled in the US and then hijack them right after the second takeoff. That's the only way this really makes sense. Keep in mind that while the flight numbers may continue on, they are usually on a different plane after landing in the U.S. For example, a British Airways flight from London to San Fran via Chicago will likely transfer to an American Airlines plane in Chicago.
Foreign flagged airlines are not permitted to fly between U.S. cities.
At least one of the Air France flights was supposed to land in Cincy - I remember that the Islamists have said for years they plan to take out the Federal prison near Lexington, KY where the blind cleric is held.
I would be weary of a diversionary tactic to close an airport, like Cincy/N.KY, which would give a terrorist plausible reason to divert to another city (Louisville) and ram into the prison instead.