Earlier quoted context omitted.
Because it's not very interesting because nobody has a good faith reason to argue in favor allowing SPAM. I assume it's being used to demonstrate the arbitrary nature of what speech people would want to allow, but it's a bit pointless. SPAM falls in the category of things that arguably are simply abuse of the system, using it in an unintended way. SPAM is not discourse, or an ideology, or a belief. SPAM is a pattern…
It's more that the SPAM issue shows that "speech people" don't have an actual working methodology of distinguishing 'good' speech from 'bad' speech any more than the very people they complain of being censorious.
That all having been said... SPAM and harassment are not problems because of the expression itself, they're problems because of the disruptive patterns of behavior. The point is not that you can't say something, or have a given opinion, or etc.
I'm not really sure how this came to be everyone's ultimate catch 22 on free expression when there's more obvious caveats, such as how arbitrary the law is. But as arbitrary as the law is, it's like gofmt. Nobody's favorite, but everybody's favorite. (This is possibly one of the worst HN analogies this month, which now that I think about it, should probably be a thing someone tracks.)