Your analogy is apt inasmuch as both the pool and the restaurant will dilute the concentration of harmful substances to the point that only a hypochondriac would worry about it.
But then I married a nurse who began to point out that certain of my health problems were related to the toxin, nicotene. So, I told her she was wrong, and went to my doc for proof. He confirmed what she said, so I decided to stop for awhile. The health problems (mostly cardio-vascular) went away.
The fact is that 500,000 people die in the U.S. every year from cigarettes. If they simply died, that might be one thing. But they linger. And we pay for it in the form of lost productivity and higher taxes (to pay for their health-care).
Anything to discourage people from nicotene is good.