> we're only interested in the risk , independent of secondhand smoke, of someone choosing to smoke getting cancer.

No, that's where you're wrong.

You are only interested in that independent risk.

I, and many others, are interested in how much smoking changes that risk.

Picking random numbers, let's say smoking gives you a 10% chance of lung cancer. It's fine for you to only care about that 10% number, you get to care about what you want to.

But for the rest of us, when making informed decisions based on risk, it matters whether smoking changes it from 9.9% to 10%, or 0.1% to 10%.