Because we are forced to breathe second-hand smoking that produces lung cancer.
Here's some info from Health Canada on second-hand smoking:
http://www.hc-sc.gc.ca/hc-ps/tobac-tabac/second/fact-fait/index-eng.php
The people who bring up the smoker's right to smoke outside have it backwards. It's not about a smoker's right to smoke in public place; it’s about the public's right to breathe smoke-free air
Ultimately, I believe it will depend on how you look at it: their right to smoke in public places, or the right to give you lung cancer among other lethal diseases.
Please take action! Help us make Canadians healthier.