I don't think it's weird at all unless I'm also weird. Fire (plus earthquake in the case of Jpn) is one of the first things that come to mind as to why I would never leave my kids alone in the hotel room. Those are things that can kill them if there's no one to protect them. There are serious building fires in Jpn all the time. Even if it's not a legitimate fire, what if the fire alarm goes off? What would be the fallout if you leave kids alone in the room in such instance?