Webster's Dictionary defines harm as: to hurt, damage, injure. People who don't or can't think believe that government should step in to prevent one person from harming another, such as in the case of tobacco smoke.
But harm is a two-way street, and it's a daunting task to determine whether one harm is more important than another.
Let's list just a few instances of harms and decide whether they should be banned. When handheld calculators were invented, manufacturers of slide rules were harmed. They were run out of business. When chain hardware stores like Home Depot and Lowes opened, many neighborhood hardware stores were run out of business. When I married Mrs. Williams, other women were harmed by a reduction in the number of highly desirable men to marry.
I enjoy smoking, and you might find it an abomination and worry about the health effects of secondhand smoke. If I'm stopped from smoking, I'm harmed by a reduction in my pleasure and you're benefited. If I'm permitted to smoke, I'm benefited and you're harmed.
There are literally thousands of examples of how people harm one another. No one but an idiot would make an attempt to objectively determine which harm is more important than the other and should be banned by government. Thus, we're confronted with the question: What is it that decides what kinds of harm should be permitted?
In a free society, the question of who may harm whom in what ways is decided through private property rights.
Harming another by rape, murder and robbery should be prohibited because it violates private property rights. We own ourselves. Thus, rape, murder and robbery are private property rights violations.
In the case of the handheld calculator producer harming the slide-rule producer, it's property rights that decides. The calculator producer owns his materials and skills. Customers have private property rights to their money. That means they have the right to spend it purchasing calculators. The slide-rule producer has no right to force customers to purchase his product. While the calculator producer has a right to harm the slide-rule manufacturer by offering a more desirable product, he doesn't have the right to harm him by burning down his factory.
What about cigarette smoke harming others? In a free society, as opposed to a dictatorship or mob rule, the matter is resolved through private property rights. If you own property, be it your house, restaurant, airplane or workplace, another does not have the right to smoke on your property without your permission.
Alternatively, in the house, restaurant, airplane or workplace that I own, another doesn't have the right to prohibit smoking. If you don't like the fact that smoking is permitted in my restaurant, you can go elsewhere. Similarly, I can do the same if you don't permit smoking.
Of course, if there's dictatorship or mob rule, and I can get the ear of the dictator or mob, a law can be written to require you to allow smoking. You say, "Williams, that would be unfair." It's no more unfair than when people get the ear of the dictator or mob and get laws written to ban smoking.
A free people will always want private property rights to decide who may harm whom. It's less arbitrary, more certain and less subject to political whims.