A) Second-hand smoke hurts other people, not just you. How far from your workplace doorway do you stand when you smoke, hmm?
B) The damage you do to your body with those cigarettes costs other people (people who use health services), your employer (if they subsidize your health insurance), and your doctor time and money that wouldn't have to spend if not for your selfish decision.
C) Is your job allowed to tell you that they won't hire heroin addicts? Are they allowed to fire alcoholics who are drunk at work?
To some degree I agree. If you were using a nicotine vaporizer or something and it didn't impact your work then it is not another person's business.
Likewise if you only smoked at home, had a clause in your insurance that said they wouldn't cover you for smoking-related illness (but covered rehab), and doctors were allowed to just let you die so they could spend their precious time elsewhere then it wouldn't be anyone else's business either.
In practice it's probably dumb to just outright fire smokers, it would make more sense to pay for part of their rehab and make it a requirement to continue working.
And of course they'll likely need something else to help relieve stress.
You're an addict. You need help. I hope that one day you do yourself and your loved ones a service and get help for it.