Think about this: YC is a business. YC likes hackers -- hell, YC are hackers. Gaming systems is what hacking is about. Hacker News provides a platform to let their companies get attention. Being front page on HN brings you traffic and tech attention. The HN that you read is the same one potential hires, investors and journalists read.
Not only that, there's a variety of mechanisms built in to boost/identify YC users to other YCers to provide mechanisms of boosting their submitted stories. HN in itself is like a giant voter ring in that way.