Does your boss have a right to demand that you behave a certain way on your own time? For example: completely forbidding tobacco use?
Some people argue that this is part of your boss' right to hire who he wants. I am not disputing that right. What I would warn against is the slippery slope of where this leads. Under this scenario, you would be the property of your boss.
You only sell your time to your boss, or looking at this another way, you rent him your body for a certain amount of time each day. He has no claim to your body during the time he is not renting it, nor can he demand to control the time you did not sell him.
Yes, your boss has a right to hire whoever he wants, but if he discriminates in this way, you and I have a right to refuse to do business with him. Any business or dealings. In a free society this could mean shunning him to the point of starving him to death if he chooses to remain a bigot and enough people care.
If a boss wishes to assert ownership of his employees, he is behaving in an evil way. Slavery is still wrong. Your body chemistry is none of his business as long as you are not contagious or radioactive. Unless the substances leave your body and make you into a hazard it is no one's business what you contain, be it nicotine, THC, or whatever. Making your body chemistry a condition of employment is wrong, but still within his rights. It is also a good way to judge the boss' character.
Your boss' rights do not cancel out your rights. You own your own body and life or you own nothing. You can choose to sell yourself to an authoritarian boss. I hope you don't. If fewer people were willing to sell themselves (as opposed to renting themselves) then fewer bosses would be able to assert ownership over their employees. This would increase real freedom even more than decorating lampposts with swinging predators. Who do you spend more of your life around: the boss or a congressman?