The 5th and 14th Amendments to the U.S. Constitution guarantee all people equal protection under the law. An equal protection challenge to a smokefree ordinance is based on the theory that an ordinance discriminates against either smokers, or businesses because it does not treat everybody the same.
Courts, when faced with this argument, look to see two things- if there is a rational basis for distinguishing between groups of people, and if there is a rational relation between the ordinance and a legitimate government purpose. No court has ever found smoking to be a fundamental right, and therefore, has never found that distinguishing between smokers and nonsmokers infringes on any fundamental right. Moreover, courts have found that protecting people from exposure to secondhand smoke is a legitimate government purpose.
It is important to limit any exemptions to an ordinance, thereby lessening the chance for any equal protection arguments to be made by opposition.