Jakob Nielsen wrote in 1998 essay Microcontent: How to Write Headlines, Page Titles, and Subject Lines that headlines on web pieces need to be different than on print pieces because they are read out of context with the print article and because there is no supporting data, such as photos, logos, etc, around the headline to help readers understand the headline.
Because of these differences, the headline text has to stand on its own and make sense when the rest of the content is not available. Sure, users can click on the headline to get the full article, but they are too busy to do so for every single headline they see on the Web.
Revisiting this topic this month, Nielsen says while active voice is better there are some occasions where passive voice might work better.
Simple sentence structure, active voice, and positive statements have been key Web-writing guidelines for more than a decade. I don’t want you to abandon these good ideas. They do improve content usability in most cases, particularly for body text.
However, recent findings from our eyetracking research emphasized the overwhelming importance of getting the first 2 words right, since that’s often all users see when they scan Web pages. Given this, we have to bend the writing guidelines a bit, especially for elements that users fixate on when they scan — that is, headlines, subheads, summaries, captions, hypertext links, and bulleted lists.
On Boing Boing the article has generated a discussion about Nielsen and his views.