I don't know that it would matter whether it could have actually been demonstrated, in court, that the airlines were liable for the losses. With 3,000 grieving families looking for someone to blame, and any number of lawyers looking to cash in if they could get the airlines found liable, suits would have been filed, and the airlines would then have been obligated to defend those suits, which would have hit them hard with legal expenses, discovery expenses, and so forth, right when people were unlikely to be buying tickets, being afraid of another attack. How bad would the damage have been? There's no way to know; if it didn't destroy the airlines, it could easily have wounded them greatly.
This argument isn't convincing. Airlines have been bailed out -financially- several times before. If a suit happened to crush them, yet another infusion of taxpayer money to a critical piece of national infrastructure would have been made.
Moreover, if the govt. was actually concerned about airline liability, they could have done for the airlines what they did for the phone companies; granted retroactive immunity for any actions taken by the airlines up to 9/11.
