This is the conundrum that Matt Simmons noticed during a period of being a frequent flier. His blog post details brilliantly just much airlines hammer it into your heads to not smoke … and then the confounding ashtray in the bathroom (with a no smoking sticker on it, no less!)
So why would they do such a thing? It turns out that having an ashtray in the bathroom an aviation requirement, based on human behaviour. After a 1973 flight crashed with 123 fatalities due to an improperly discarded cigarette, airline authorities realised the need to ensure that anyone wanting a cigarette would dispose of it safely. Even now. So if you do smoke in a plane, you’ll probably get booted off and fined, but at least you should be able to get rid of it without endangering the safety of the plane.
Simmons suggests that this is all about affordances (and if you don’t know much about this, you should check out Don Norman’s Design of Everyday things for more), and behaviour (he recommends reading Nudge if you haven’t already). This story reminds me of the behaviour encouraged by the infamous Schiphol airport fly urinals as well.
That said, there is one less place where you’ll see that sign. Notice how the one above your seat never goes off? Some modern planes now use that space to tell you when you can have your electronic devices on and off.
Image NC-CC by roychung1993