ycombinator ain't doing this as a charity, mate.

it's here to increase visibility of the ycombinator brand, it's startups, and the technologies related to what they do. discussions around it were going to happen anyway, so own those and shape.

create an ecosystem, and then mold / skim / ingest / bask in it.

it's the same reason why BigCorps try really, really hard to get ownership / mod control of subreddits, and/or having their marketing assets responding to discussions in their sub or elsewhere.