In certain cases they may not be able to fire you, nor wish to, for violations of rules you agreed to to when taking employment. However, they may curtail your advancement, offer training or assistance, or just write you off and ignore you. If you are a critical, highly valued employee, well skilled, dependable, etc. they may choose to overlook what they perceive to be your negatives.
Some of the "what ifs" you mention are federally protected and a company would need to find other reasons for termination. Usually not hard to do.
If you agree to certain restrictions on your private life, smoking, upon employment then you've agreed to it, live with it. Simple! The company doesn't own you, you can exercise your right and seek employment elsewhere should you find the rules too much to for you to handle. This is usually only an option in areas where employment is high and companies are strained to find employees. Or, you have skills which are highly sought after. If you are a truck driver, you'd better be a damned good one. Not disparaging the truck drive, just an example.
Bottom line, net profit, is determined by many factors, profit margin being only one of them. The employee is not the one who determines if a profit is acceptable or good, shareholders (owners) do. An employee is simply a hired hand. As gloucesterman so ably pointed you are offering your skills and time for a price. The employer determines the value of your services. A lot of us value our services higher than the market will support.