Why wouldn't you make a post on HN to try to get some extra eyeballs? It isn't like you can pay to have posts on the front page - you still need support from viewers to move it up top.
TechCrunch is a different issue. Yes, cross-posting everything here through ghost accounts is a sucky move, yet their subjects are "relevant" (when it's about tech, silicon valley life),...
And I wouldn't go as far as say "People don't post on HN for extra eyeballs", but there's a difference between a member advertising a tech product and someone who creates an account just for advertising a non-tech product. If not, what's next? Anybody here trying to sell his car?
More power to you if you can get people to up-vote your ad for a 1992 Toyota Corolla without people flagging it into submission.