There isn't much incentive to auto-spam a site that not many people know about; It's much quicker just to have an intern do it manually, if that's the way you swing.
The thing is, News.YC is a pretty small and focused community.. There's not much advantage to robo-spamming it. It'd be obvious what you were doing, and the people who were reading it would just ignore you. Since you'd likely be the only fool to attempt it, PG would probably notice and just ban the IP.