If the battery spontaneously combusts in the hold, you have a serious problem. The fire will be quite large before anyone knows about it. If your laptop catches fire in the cabin, you'll probably scream immediately, and they have burn proof fire bags to shove the laptop into.
Many airlines also wont allow shipments of lithium or things containing lithium batteries on passenger flights. Apparently they have a habit of spontaneous combustion. So shipments of iphones have to be specially packaged and sent on cargo flights.