I don't know HN's rationale, but: if you try to ban self-promotion, then in practice you're only banning honest self-promotion. And if you allow it, but try to ban accounts that only exist for that purpose, then in practice you're encouraging them to spam mediocre comments in other threads to dilute the signal. [edit: I see that the guidelines do say not to use HN primarily for promotion. I'm willing to bet this significantly increases the amount of insincere engagement, but obviously the admins/moderators here are more aware of the tradeoffs than I am, and they have decided they are worth accepting.]

IMO the important thing to try to crack down on is vote manipulation; if you have that under control, then you can probably happily allow self-promotion and trust that only the interesting stuff will rise to the top.