In Michigan and 19 other states, employers have the legal right to fire anyone, as long as they don't violate discrimination laws (for age, gender, race, religion, disabilities, etc.).
There may be no rights conflict but there's a definite double standard.
In a truly free nation, there would be no such thing as "anti-discrimination" laws in the first place.
It is the same private property rights issue discussed on nearly every FR smoking thread. The company is the private property of the employer. Employment is not a right. Employment is offered as a means of improving an employer's capacity to do business. This employer has a single standard. Employees may not smoke. If that is unsatisfactory, the employee is free to seek employment elsewhere.