Usually they'll change submission titles to match the original article's title.
I actually prefer it this way. This way, HN is neutral - by presenting the article title as-is, they aren't asserting any opinions on the subject (usually).
The goal is not to reflect anyone's opinion. It's to make HN a more useful and interesting site, which isn't going to happen when the tendency is to make article titles more boring and generic -- resulting in most users not even bothering to read the articles.
Why is the author's opinion more relevant or acceptable than the submitter's or HN's? You state it like it's obvious, but I don't see why the author's opinion is particularly special.
The way I see it, when I click a link, I'm signalling that I'm interested in reading the author's opinion, not the submitter's interpretation of the subject - that's what the comment section is for.
When I put it that way, it only makes sense that the title should reflect the original article's title.
When I'm reading through a list of titles on HN deciding which to click on, I'm more interested in why the submitter thinks it's relevant to HN and why they think I should click it. If I do click through, I will read the title.
I actually prefer it this way. This way, HN is neutral - by presenting the article title as-is, they aren't asserting any opinions on the subject (usually).