Why Y Combinator censor smaller blogs and not bigger spam ones?

Even if the content is just as good if not better on the smaller less traffic intensive or well known blogs? Are they not allowed to post here?

Is there some sort of a deal where if you pay Hackernews or just by virtue of being a popular site that spams links all over the internet they allow you to post unlimited amounts of links here without the worry of getting put in a blacklist filter? While if you aren't a super popular site you basically aren't allowed to post up even one link a month and if you do you get permanently blacklisted?

Honest questions