Apparently you're to take laptop batteries into the passenger cabin because fires are easier to deal with in a metal tube full of people.
"In May, a spare li-ion battery stowed away in a passenger's computer case, neither entombed with insulating tape nor placed in a separate protective case caught fire. A flight attendant was able to remove the burning case from the passenger cabin and toss it onto the ramp."Which is clearly fuck all use at 30,000 feet.
One has to ask, just what would they have done when a metal / chemical fire breaks out during a flight? One can only hope they have a class D fire extinguisher on board, since they can't just open the door and pitch the burning laptop out!