Smoke-free businesses are a gray area, because we're talking about an addictive substance. Businesses that refuse to serve tobacco addicts are at a disadvantage that apparently can't be overcome by additional patronage from customers seeking a smoke-free environment. So the government steps in, in the name of public health. I don't agree with these laws 100%, but I can see the argument behind them.

Full-service gas stations, on the other hand, are neither prohibited nor mandated in most states. They would exist if there was a market. There's not. End of story.