It becomes headline news because the person was famous. Whether we like it or not, it naturally follows that when a famous person dies, it makes the news - the cause of their death usually makes the news as well, because people are curious.
As for surviving family members - surely you are aware that Ms Fisher was quite open about her substance abuse? That she titled her autobiography "Wishful Drinking" should tell us something. It isn't as if this was a "dirty little secret" when she spoke openly about her substance use/abuse.
I just think it's a shame that people are so desperate for random pieces of information about those who are/were famous for something unrelated. What I see is "Carrie Fisher died" and from there I don't see why it matters whether she died in a car crash, of old age, natural causes or something like substance abuse.
The fact she didn't keep it a secret doesn't mean it needs to be splashed all over the newspapers after she died.
So in your opinion the moral thing to do is pretend her substance abuse didn't exist, even though she spoke openly about it - and anyone curious if it may have contributed to her death is on a "relentless quest", covered by shame for inquiring about something so morbid - or simply connecting a few logical dots?
I don't know about anyone else - but I don't think less of her because she had substance abuse problems or because they were tied to her death.
I'm not sure where I said the moral thing to do is pretend she didn't have a problem with substance abuse.
She was open about her substance abuse. Someone who has issues with substance abuse who suddenly dies may have died as a result of substance abuse. Whether or not Ms Fisher's death was caused by substance abuse isn't really any of my business. It's not as if it's a secret that substance abuse can lead to the premature demise of the abuser, so it's not as if being able to point to her and say "look, that's what happens if you abuse drugs" is a particularly useful thing to say.
I haven't read her autobiography so can't comment on what she had to say about her issues with substance abuse but I'd hazard a guess it would do far more to honor her memory to look to initiate adult discussion over the social, moral, legal and economic aspects of substance abuse in general than to focus on whether or not it was specifically responsible for her premature demise.
Here's the thing. Whether or not drugs/alcohol/whatever killed Ms Fisher makes no difference to my life or the life of anyone else. It's a single data point that provides information with little context and less value. If it were to lead into a discussion on what to do about the wider problem of substance abuse maybe some good can come of it and I suspect, given she was open about her issues, she would be quite happy with that. If all that happens is another data point gets logged of "star takes drugs and eventually dies" then it seems like an opportunity wasted, no?