I will probably be told that I'm an idiot but I'm not sure that they have actually come up with definitive proof that ties the two together. Yes there is overwhelming circumstantial evidence that smoking is responsible, that the carbon monoxide, tar and the hundreds of other poisons and toxins which are present in the smoke are responsible, even the tobacco companies have more or less conceded the link but I'm pretty sure that an absolute connection has yet to be established.
I am in no way trying to justify or support, defend or diminish either the act of smoking tobacco or be in denial about the end result of smoking tobacco, I know probably millions of people have died because of smoking related diseases, not just cancer, so please don't shoot the messenger I'm just answering a question and if I am wrong and they have actually published a definite link then I would be more than happy to admit I was wrong!