Your argument ignores nearly all of the factors involved with dosage -- most notably tolerance. Tolerance reduces perceived effectiveness, requiring increased dosage to achieve the desired psychological results. People regulate their consumption to that psychological effect, but the negative physical effects do not share the same curve.

Not to mention the vast difference between what it says on the label and what you may actually be consuming -- the testing and labeling processes are rather notorious for being inaccurate. Even for an avid smoker, it's virtually impossible to consume the exact appropriate dosage, if there even were such a thing. There's also social pressures which encourage people to consume larger amounts. The list goes on and on.

So, what you're suggesting as being the obvious negator of the expert opinions of medical professionals, simply has no foundation in reality.