You're breathing in car fumes all day, a bit of smoke from 5 meters away isn't much worse.
If you break your leg, is it okay for someone to punch you? I mean, it's way less damage than you've already sustained, right...
Smokers should be considerate, that's a given, but you're arguing a false premise with hyperbole.
You're overcomplicating it, it's not about that, it's simply fuck them for flagrantly smoking right in my face. It's literally a physical assault, and entirely "optional" / their choice to not give a fuck about anyone else.
There's a huge range of possible things like this people could do to each other which is simply not okay out of basic consideration for others / not being a massive dick. What if I decided I like the smell of some other chemical that's bad for you / proven carcinogen etc, and open this up on a busy train platform. Do I then get to argue technicalities about other unhealthy things too? What if everyone did this?
Is this the point we've reached in society, where we want to argue our right to do such things to each other? Purely out of choice, not even some necessity...
An imperfect world is not an excuse for inaction
In the developing world, where very old cars are routinely used, the fumes are worse than smoke from individual smokers. In the EU, it is the other way round.
How come that nations that have more diesel engines don't suffer from cancer more?
In Ireland or Italy, something like 40 per cent of new cars are diesels. In Finland it is less than 20 per cent and in the Netherlands less than 10 per cent.
If diesel was a cancer machine, I would expect Italians to have a lot more cancer than the Dutch. Dose-response is a thing in toxicology.
Honestly, since they banned indoor smoking and having people smoke 9m from doors, I don't mind it. What I do mind is people throwing butts all over the place, for some reason, even people who avoid littering don't think twice about throwing their dirty butts everywhere. Every time spring comes and snow starts melting it's so disgusting everywhere.