Earlier quoted context omitted.
Just think of all the uncompensated value you've given Y Combinator every time you've left a quality comment on their forum.
Then again, Y Combinator makes no direct money off HN. AWS’ docs, if they are quality directly impacts their profit because more people can use their services more easily.
1) Special ad positions with front-page placement (currently #22 is "Willing (YC S15) Is Hiring a UI / UX Designer" https://willing.com/designer , a privileged post that can't be voted or commented on)
2) The moderators can and sometimes do intervene by removing certain comments that are critical of YC companies.
Comments make the forum more interesting to the community, which increases eyeballs and makes the front page spots more valuable. And shaping the discourse helps with damage control when necessary.