Because rules impose restrictions on peoples ability to use public space. The entire point of public space is that nobody owns it, so everyone has free and equal legal access to it.
So what you're saying is that anyone can do whatever they want on public property? Because if that's the case, why can't people drive motor vehicles through parks, write graffiti on walls, or (usually) sell items from vendors? Most public places are in fact owned by something or someone, whether that be some wealthy individual, a company or organization, or run and maintained by a city or jurisdiction.
You have it backwards. In a scenario with smoking banned, it's the ones with lung issues getting the special treatment and entitlement. Lung issues are a personal problem and it's up to the individual to care for their own health (i.e. avoiding smokers) rather than society as a whole to pander to everyone's potential issues.
Freedom is never a "special treatment" or "entitlement." It's fundamental to humanity.
And what if I said you have it backwards? A lung issue is usually not the individual's fault (I'm not saying you said that), but smoking is entirely up to the person doing it. What I'm getting at is that smoking is a decision, but something like bronchitis is not. What I don't understand is that people with lung issues should have to leave a park because some smoker decides he wants to use the particular spot that other people are using for himself.
It's like having a group of 100 people all in a fairly tight area (e.g. a line to go into a store) and one person in the line decides to light a joint and smoke away. Well I guess that means anyone with an unavoidable condition must now leave the line cause that smoker wants to enjoy himself.
And I'm not saying that smokers should be removed from the public altogether. I think they should just go somewhere with fewer people. I see employees on break who will go outside the store where they work and just stand somewhere out of the way to smoke. I don't see any issue with that.