B757 Captain reported concerns with the deferral of lost lithium battery powered electronic devices lost in the aircraft.

Narrative

The 757 fleet (and possibly others) has a deferral that allows the aircraft to operate for a specific number of cycles if a passenger cell phone or other electronic device falls down a gap in the cabin sidewall into the cargo pit and cannot be located. Maintenance usually tries to look for the phone; to include disassembling some of the cargo pit panels - but if they can't find it in a 'reasonable' amount of time then a deferral is created and additional flights are authorized.The FAA website says this about devices with lithium batteries: 'Devices containing lithium metal batteries or lithium-ion batteries; including - but not limited to - smartphones; tablets; cameras and laptops; should be kept in carry-on baggage. If these devices are packed in checked baggage; they should be turned completely off; protected from accidental activation and packed so they are protected from damage.'We have provisions for Dangerous Goods being shipped as freight; being used by crew on the airplane; as part of installed equipment; and being carried by passengers or crew on their person; in carry-on bags; and in checked bags. There are no Dangerous Goods provisions providing for lost dangerous goods floating around the airplane somewhere.The concern here is the potential for damage and subsequent fire. We won't even let a passenger move a seat if they drop their phone in the seat - yet somehow; it's okay to drop a phone into the cargo area where it could easily be damaged by the cargo loading system or something else? How is this safe? Or even legal? If a lost device with lithium battery gets crushed and catches on fire; we are not going to be able to fight that fire.I'd like to know specifically what kind of risk analysis (if any) was done for this deferral.