Lithium ion cells can and do self destruct for no other reason than a manufacturing defect coupled with a physical event. Remember the big Sony recall?
When you are in a plane, it's not like you can just throw a flaming battery out a window or lock it safely away quickly. You are in a closed environment and what anyone will tell you, nothing is worse on a plane than fire, especially one not easily extinguished.
Yes the odds of this happening are rare, but there are also a lot of flights every day. It may be rare for something to happen on any given flight, but to happen on any flight over a year, suddenly the odds do not look too bad.
Passengers: Cannot check Lithium Ion batteries -- cause if they start on fire, there goes the whole baggage compartment. Yes there is fire suppression in baggage, but again not what you want happening. There is also a limit on the size of batteries. As has been noted, they spread from cell to cell. 100wH of batteries can be contained. 1000 perhaps not.
Cargo: Banned for the most part, see above. Fire spreads from battery to battery and if you have a lot of cells together, it only takes one failure.
Life will go on without planes shipping large amounts of LiIon batteries in cargo, hence prudent thing is to stop doing it as it has been proven they can start on fire and the results dangerous.
To the person who equate the dangers of Lead Acid and Alkalines ... Lead Acid are only dangerous during charging or puncturing. They don't randomly go off, and neither do alkalines.
