Please tell me that once back on the tarmac, the passenger realizes that he was sitting on it!

This is due to the possibility that the lithium battery could be crushed, say in a seat recline mechanism, and catch on fire. Your modern small black rectangles are actually quite dangerous in a confined space.

My guess is it is probably due to the risk of fire if a lost cell phone gets its battery damaged in a reclining seat mechanism or other moving part.

Thats a reason why you dont use only e-wallets.