Fires on planes are a little more serious. If not stopped quickly, they can spread through the space between the fuselage and the cabin, filling it with smoke. At altitude, they don't have enough oxygen to burn that well, but if it's spread far enough, as soon as you're on the ground the inrush of oxygen will turn it into an inferno in about two minutes (this is why manufacturers have to be able to show that the plane can be entirely evacuated on something like 90 seconds with half the exits blocked.)
Where I've lived, there are ash trays everywhere, not despite; because of laws prohibiting smoking in these areas. The idea is to provide a safe and convenient place to dispose of your cigarette now that you've entered a non-smoking area.
