I think the first implementation of the black bar was for Steve Jobs. Well, I'm not a fan of Steve Jobs, and I think that individuals like Maryam Mirzakhani are far more important in the course of humanity than Steve Jobs ever was. But I don't question the decision to put the black bar for Jobs: it corresponded to an emotion, felt by a lot of people here and more importantly by the owners of HN. It was their way to express their emotion, other people wrote blog posts, tweets or Facebook status. But it was all a question of emotion, and the merit of the person was secondary. The black bar is like the black clothes of people at burials or the black veil of the widow: it's there to express sorrow, not to mark how much the person was a great scientist or entrepreneur. And who are we to judge emotions or impose them to other?
I don't think there should be a specific criterion. Putting on the black bar now when it was not done spontaneously is like faking a deep sorrow that do not exist.
I could not agree more with you about these discussions, which occur at the bottom of every thread about any recently deceased, like a sort of ghoulish, amoral Slashdot "first post" contest. It is not OK to litigate how strangers choose to relate grief, least of all in their own damn house.
The easiest way, then, to understand the "black bar" is to think of it as an expression of sorrow that has a direct connection to HN's operators.
Much the same way a family remembers their loved ones, but doesn't grieve every important person who has passed away irrespective of their stature. Like parent was saying, it's a little discomfiting to see this discussed on threads.
Edit: I just did some digging around and it seems they didn't have a bar for Neil Armstrong's death either, so this seems right on the mark so far.
But then, we can see your tantrum over the matter above..
That said, I understand how HN is not really a Math/CS forum, it's more about industry.