If you don't do that you're on the other end of that slippery slope.

Should a chain smoker and someone with no elective risk factors have equal eligibility for a lung transplant? They're otherwise comparable patients. One's going to die, and there's only one lung to go around.

We seem to survive as a society by using systems like that for insurance that isn't health insurance. If your house burns down it matters if it was because of foreseeable and reckless behavior on your part.

Except it’s easy to prove negligence with a house fire, less so for medical problems. But those aren’t equivalent things. A fire is a one-off event, and if you don’t catch it right away, it runs its course and then it’s over. But how do you adequately paint a smoker as negligent? They’re certainly doing it to themselves, but they also have an addiction, propped up by companies and a society and probably a social group that supports them.

It’s obvious whether or not you’ve had a house fire, and it’s sometimes possible to divine who was at fault. But healthcare isn’t binary like that. Organ transplants are a special case too, and the ethics there are tricky because of limited resources. It also depends he wilt on compatibility. It doesn’t really matter if a smoker gets new lungs and a non-smoker doesn’t if the smoker is compatible with available lungs and the non-smoker isn’t.

Are you really responsible for your car accident record, or did you through no fault of your own develop an adrenaline addiction, propped up by auto commercials and a neighborhood that normalized street racing?

For any tax or public policy you can find sob stories on the edges, it doesn't mean it's not a good idea in the aggregate to enact the policy.

Which is what I'm pointing out in this case, it's generally a good idea to tax the specific harm that's being addressed (unhealthyness), not some mostly-but-not-quite related variable (sugar consumption).

That organ transplant example isn't meant to be plausible, but an apocryphal story. You presented a story of "no treatment for you" for someone being grossly negligent with their own body.

I'm pointing out that you'll equivalently have otherwise healthy people who don't get timely treatment because the health care system overall is strained by obesity-related illnesses, tobacco-related cancers etc.

Doing nothing is equivalent to continuing to punish people for their health problems because some other people have preventable self-induced health problems. So I don't think you can make the slippery-slope argument, or claim the moral high ground.