That's actually true, but not in the sense that most people mean it. Usually when I hear this it means something like "HN promotes YC's business and suppresses criticism of YC", phrased in more sinister language. But that of course would be the dumbest way to operate such a site, especially with this crowd.
The way we promote YC's interests is simply by trying to keep HN interesting. The most interesting HN is the most valuable-to-YC HN. That's why people come here, so that's all we optimize for. Literally everything we do with site moderation derives from that.
For example, why do we moderate flamewars? Not because we secretly agree with one side or the other but because flamewars are repetitive and therefore bad for curiosity. Plus they encourage people to do worse things, so their damage extends forward in time. These are empirical observations, not moral edicts. We just look for what makes HN richer and more complex vs. what makes it dumber. Scorched earth makes it dumber.
I remember years ago hearing a radio interview with the CEO of Costco. When asked, "What would you say to Wall Street analysts who criticize you for giving all those benefits to your employees?" he replied, "I would say have you ever run a major retailer?" Something similar is how I feel when people criticize how we moderate HN. The trouble isn't the criticism—I'd be delighted to be shown how to do this better. The trouble is that the criticisms usually aren't relevant to the actual problem of running HN, which is: how to maximize the interestingness of a public-anonymous internet forum dedicated to intellectual curiosity. Maximizing interestingness includes preventing decay.
I hope that doesn't sound complainy. We're all lucky to have the opportunity to participate in an experiment like this. It derives from two things, btw: pg's own curiosity, which started the site, and the fact that the people running YC get HN's core value. It's an unusual set of conditions and we're all responsible for making something interesting of it.
p.s. I forgot to add that there are also some formal ways that HN gives back to YC: https://news.ycombinator.com/jobs and https://news.ycombinator.com/launches.