In my youth, I loved to watch my neighbor light his grill. He would douse the charcoal in gasoline...wait a minute for the fumes to build...and then THROW a lit match at the grill. It would flare up in a large ball of fire.
And I'm pretty sure that just about anything that is 'on fire' will ignite gas...you throw anything burning near evaporating gas, its going to ignite.
A cigarette, however, isn't typically burning with a flame. So, it normally wouldn't ignite gas.
So, why is it a bad idea to light up near gas?
1. That lighter people use to light the cigarette...well that is certainly a flame.
2. If a 'cherry' from a cigarette falls off, it gets very red on the way down, as its descent is the same as fanning it with wind...and when it hits, small sparks fly around. I would not bet my complexion that those sparks wouldn't light gasoline.
No argument from me.