You've got to organize a voting ring. Sadly, I've seen this work quite well for mediocre submissions several times. I've seen YC companies do this.
That being said, there are some YC companies that do this. However, the flag mechanism far counteracts bad articles which have received unjust votes, so it's an unstable equilibrium.
As for YC company voting rings: certainly this exists, but we penalize them too. I can't claim that we do it perfectly even-handedly—that would be claiming no unconscious bias—but we try to. That's one reason we do as much as we can programmatically, since the software has no such bias.
The principle governing HN is to have the best possible content on the front page. Anything else would be pound-foolish. Not that we do have the best possible content, but that's our goal and is what drives our thinking.