The guidelines of Hacker News
http://ycombinator.com/newsguidelines.html
were actually edited to the current version quite recently, after an earlier discussion of this issue. The current guideline language is, among other details about titles,
"
. . . .
"Otherwise please use the original title, unless it is misleading or linkbait."
That plainly says that original titles are preferred.
The guidelines also say, "Please submit the original source. If a blog post reports on something they found on another site, submit the latter." So blogspam is plainly disfavored. Personally, I think that it is a rare blog that has many posts worth submitting here, and I'm always on the lookout for better sources from which to submit articles.
http://news.ycombinator.com/item?id=4624933
By the way, as a rhetorical matter, I'm surprised after reading through all the other comments before posting mine that I don't see a lot of examples of before-and-after titles to see if the submitters who don't like the curators preserving original article titles (or shortening original article titles in a way different from the submitter) are really coming up with titles that are much better than the original article titles. What are the most important recent examples you have in mind?
My own experience on HN is that when I see a cool article from a good source, and I submit it with the original, professionally crafted title, I sometimes submit an article that was earlier submitted by someone else, and sank into oblivion because it was retitled in some way that made the article look dumb. Few participants on HN have much professional experience in headline writing, and I'd rather have most submissions be submitted with their original titles based on my observations. (If you have convincing counterexamples, that is examples of HN-user-made titles that were really good and more helpful than original article titles, I'd be glad to consider those examples.)