I think that this is down to the Guardian's politics - cultural relativism means prioritising one piece of writing as "news" over another as "opinion" is invalid.

I've got to say, having watched the journalism sausage factory in action while running a newspaper, I'm not entirely opposed. Journalists routinely choose what sources to cite and which to ignore. News is often, if not always, manipulated to communicate a political point of view. Dropping the pretence and presenting all news as opinion (or all opinion as news) seems at least honest.