To expand on what OrphanWilde wrote:

The Just World Hypothesis can be summarised as "you reap what you sow." If you wish to argue that you don't "deserve" to reap what you sow (perhaps because you didn't have access to better seeds), or that it's not "just" to reap what you sow (because everyone should reap in rough equality, regardless of how they sowed), or similar, that's fine, but you aren't arguing against the Just World Hypothesis.

So when we see the fruit, the Just World Hypothesis tells us: that's probably how the person sowed the seeds. And yes, there is noise, which is why it's a heuristic, not an infallible rule. But the whole reason to sow the seeds in the first place was to cause them to bear fruit. "Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?" In other words, Coherent Extrapolated Volition.

So to take an example from the original post - smoking. If I meet someone with lung cancer, the overwhelming likelihood is that they are responsible for their own problem, through smoking. But if I smoke and then I get lung cancer, I'll want to make excuses for myself, and will stubbornly refuse to make the connection between my own culpable past behaviour (the sowing) and my present misfortune (the reaping). People who complain about the Just World Hypothesis want me to extend this non-judgemental behaviour to everyone else. But just as with the Fundamental Attribution Error, the problem is not that I am being too harsh on other people, but that I am being too easy on myself. I am right to draw the connection between behaviour and outcomes for everyone else, and I should do the same for myself.