Smoking on planes used to be commonly allowed. But now it has become almost totally forbidden.
Smoking on planes is now the exception more than the rule. Smoking within the cabin, while walking in the aisles, or while using the bathroom is not only forbidden but many countries now have specifically designed laws to punish people who smoke in bathroom. This is done because smoking on a airplanes can easily cause a fire. For example, think about flights from Los Angeles to Leon: it would only take one passenger casually flicking his cigarette end into the waste paper disposal bin to cause a fire that could endanger every single passenger.
With modern workplace health and safety requirements there is an increased reason for no airlines to tolerate smoking. If they did then their cabin crew would be forced to smoke the second hand cigarette smoke. Unlike people who work in bars, the smoke could not be filtered out by high power air conditioners as the air on a plane simlpy recalculates. So as well as being an problem regarding starting first it's also a problem from the health and safety stand point. And so it should be; cabin crew have the right to a safe working environment. The fact that their work environment is on flights from Denver to Rome or flights from New York to Port-of-Spain should make a difference to whether or not they need to endure poor working conditions.
There are airline companies that want to make smoking on flights legal, and they safe that it can be done very safely. Some airline operators in the United States are putting on flight that specifically allow smokers to smoke while they fly, a niche group of people, but a group big enough to warrant whole new routes.