I guess there's some aspect of fire safety, because cigarettes are smoldering and can start fires. But I've been in hotel rooms where a previous occupant was smoking enough that the room smells, and it's not pleasant. It doesn't happen often now that most hotels are declared non-smoking, at least in my experience.

I don't think I'm in favor of black box smoking detectors either. I'd guess housekeeping reporting the room for smoking during cleaning and a 2nd person verifying would be enough to bill a smoking fee and that would drive compliance. Sometimes you miss a room, and customers complain, and you deal with it then. Better than the sensors said X and we didn't follow up with our noses.