You are harming your kids by making their immune systems weak by not exposing them to smoke.
"These smokers are in deep denial. They must continue to think they are causing no harm to their own bodies or their childrens bodies, otherwise they would have to stop and they don't want to stop."
You're right, of course. I have absolutely no problem with their belief that second-hand smoke MIGHT do no harm, but any rational person knows that it MIGHT do harm. So why take a chance with your kids when it's so very easy to avoid it? Don't smoke inside your home and don't smoke in your car when the kids are riding. I don't avoid ALL places with smoke -- my kids have always come to the bowling alley, which sometimes is almost blue with smoke. But that's very infrequent.
The answer is either that they don't want to be inconvenienced or they can't do it because that might go aganist their agenda. Sad.