I have some background in the air freight, and air charter biz. On most commercial passenger flights, in with the checked baggage, you’ll also find air freight. I do know that lithium batteries, are considered hazardous material, or hazmat, and are not allowed to be transported, as air freight, on commercial passenger flights.
A lot of the commercial airfreight is material for the automotive manufacturing industry. Even a small carton of those keyless entry fobs, that unlock your car doors, cannot be moved airfreight, because of the lithium button cells in them.
A form of the subject rule, has been in place for some time. The key words are checked baggage. Batteries are considered hazmat, in the luggage compartment of commercial passenger flights. Do people leave a cell phone, or ipod, in their luggage, from time to time, sure, and it slides by. Same thing with the passengers. Everyone has a lithium device on them, and it’s allowed.
This is only my guesstimate as to why it’s allowed in the cabin, but not in the belly (luggage hold). This is nothing official. Just me trying to think like the aviation business. Fire suppression. They have fire extinguishers, and people trained to use them in the cabin. I don’t believe there are fire suppression systems in the belly.
If an iphone lithium goes berserk, and catches fire in someone’s pocket, the aircraft isn’t going to catch on fire. If there’s a fire in the luggage pits, they have a serious problem.
Take it for what it’s worth.