Does an Employer have the right to discriminate rules between employees some can smoke some cant?

Not enough is known about the work situation in order to comment. Are there safety or sanitation or cleaniness issues that enter into the scenario? Do some work in different areas that must be klept smokefree? If the discrimination is done strictly on the basis of favoritism it's probably not "fair," but I don't believe that any law (that I am aware of) protects against deiscrimination based on people's smokiing habits.