People who smoke don't do it because they think it's healthy. Actually, they are all too aware of the risks that come along with smoking cigarettes, and they know how much the addiction is costing them on a practical and monetary level, too. It's just that quitting smoking can be a really difficult thing to do. Addictions are powerful.
If you thought the only time you would see smoking in the office would be on an episode of Mad Men, think again. One of the most interesting cultural trends of 2013 has been the explosion in popularity of electronic cigarettes. In some offices, employees are "vaping" while working in their cubicle.