HN is one of the few places where informed discussion is actually possible and even opposing views (as long as they are objective and well reasoned) are adding to the discussion (and often upvoted). Shutting that off just because of some unfounded downvotes or trolling in times where such discussions are extremely important is cowardly and a very sad move in my opinion. HN is better (and more important) than you think.
In any event its an editorial decision: HN is a medium/tool of ycombinator and they decided that this is necessary in order to preserve its value.