Here's a link where I answered the same question about three weeks ago: https://news.ycombinator.com/item?id=5837004 , so this isn't a new issue. In fact, PG has been blocking various bots since 2011 or so; https://news.ycombinator.com/item?id=3277661 is one of the original discussions about this.
And to show this isn't a Google-specific issue, note that Bing's #1 result for the search [hacker news] is a completely different site, thehackernews.com: http://www.bing.com/search?q=hacker+news
In general, I think PG's priority is to have a useful, interesting site for hackers. That takes precedence and is the reason why I believe PG blocks most bots: so that crawling doesn't overload the site.