If I post a comment about prostitution on HN, HN is not liable
Are you sure? And are you in a position to be making that call?
No jury is going to convict the CEO of Reddit because someone somewhere posted a comment about prostitution on a subreddit.
That's probably why Reddit eliminated basically all the subreddits related to prostitution. They're also probably less concerned about their CEO being jailed than they are about massive fines.