I'd assume it's because IOS is better than ios - and no, user's tend not to capitalize, just read any Facebook or Twitter feed/stream. The general population is poor at almost all aspects of the English language. It's also not worth making exceptions for words like "iOS" when "IOS" works equally well to the average consumer.
However, more important, I think they do it because people tend to auto post to Hacker News from the bookmarklet and similar services, and it's better to have things similar rather than a bunch of differently formatted titles. Makes it more appealing to the eye and therefore easier to read.
I would have thought that the kind of people who post on HN are going to be better at correct use of English than those on your average Facebook/Twitter stream.
Maybe 2-3 years ago, but nowadays Hacker News is just as commonly used as many news services. It's also not only about the correct use of English, many people can use English correctly but do not bother" capitalizing or punctuating since it's "annoying" to do on mobile devices - and since the people who do use Hacker News tend to be more forward-moving with technology, they tend to read and make posts on Hacker News from their phones and mobile devices which they do not bother correcting themselves on.
However, more important, I think they do it because people tend to auto post to Hacker News from the bookmarklet and similar services, and it's better to have things similar rather than a bunch of differently formatted titles. Makes it more appealing to the eye and therefore easier to read.