Smokers are not a protected class. There's no federal law, nor I suspect any state laws, that protect smokers from discrimination. Federal law protects against discrimination in hiring on the basis of race, religion, national origin, age, sex, pregnancy, citizenship, family status and disability, not smoking. A company could also refuse to hire say, cat owners or dog owners or car owner or golfers or baseball fans without running afoul of federal anti discrimination laws. I don't know that the health and insurance coverage scenarios above are addressed at all in employment discrimination law, however they may be addressed in the ACA. Discrimination in hiring someone with a STD probably falls under the ADA and is likely prohibited.
People don't have a right to a specific job or to work for a specific employer. And private employers don't have an obligation to give anyone and everyone a job. Companies are just proscribed from discrimination against specific classes of people as defined by federal and state laws. Maybe someone could make an argument that smoking is an addiction, addiction is a disease, and discrimination should be prohibited. The ACLU has advocated laws that protect against "lifestyle discrimination" and has argued that if hiring discrimination against smokers disproportionately affects one race or another, one gender or another, etc. then those practices may run afoul of current federal law. But no one has tested that reasoning in court, and perhaps the evidence doesn't even support the argument. And as far as I know no anti "lifestyle discrimination" laws are on the books anywhere in the country.