Also, in the case of HN, YC offers a service. Just like a forum is a service, this discussion and news platform is a service. It's irrelevant if it's paid for or free.
Because HN is now like Reddit, but for techno-snobs. If you don't follow the tightly defined groupthink, you'll get downvoted.
Many years of discussion groups have proven that downvoting has a chilling effect on discussion groups. Allow upvotes, and "spam" flags.
Also, in the case of HN, YC offers a service. Just like a forum is a service, this discussion and news platform is a service. It's irrelevant if it's paid for or free.