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.
