Because I like Hacker News. If Hacker News was responsible for the posts of its users, it would not exist. The value it provides for Y Combinator would not match the risk.
Also, email as we know it would not exist. The price of stamps would skyrocket as the postal service would need to hire people to read all letters in case they contained death threats. If any of the web services deserves common carrier protection, it would be twitter.
Whenever something bad happens (like here), there will always be demands for more surveillance. More central control. These demands are just, the victims are real. And the solution exist. We could have a world with only a few highly curated and monitored for pay communication channels, where death threads plus anything else our governments want to keep out would be kept out.
Anyone fighting that future are insensitive jerks unable to emphasize with the victims. Or can easily be portrayed that way.
Righteous indignation comes with a price.