I completely agree with sigisgrim's logic on this matter. Why should the non-smoker have a choice of breathing the smoke or standing in the cold/rain and/or potentially forfeiting a place on the bus as well?!

If no-one's there, then fine. If someone comes along (who isn't smoking themselves at that time) either put out the ciggie or ask them in a very non-threatening way if they mind, and take "yes" graciously. People will often say (for fear of being attacked otherwise, all too common in some places) "fine" when it is far from fine so putting it out is probably the most considerate thing to do.