It's very common to remove smokers from studies like this (anything looking at mortality). Smoking increases your chance of so many health issues: heart disease, lung cancer, COPD, high blood pressure, stroke, aortic aneurysm, general infection due to chronic inflammation, etc etc. It's well documented and pretty unambiguous. If you smoke, your risk of all the above increases dramatically. Which means as a researcher you can't leave these people in a study looking at mortality rates since they would confound the results too much.

Basically, the risk of confounding your results with smoking-related mortality is a bigger problem than biasing your population with fewer people who consume addictive substances.

Source: I'm an ex-molecular biologist, my significant other is a PA.