newsreaders and presenters have a tricky job - they must present information in such a way that the presentation itself does not detract from the content. Thus, certain accents are acceptable nationally, but localised phrasing and speech patterns are not. Neither must speech be too precise, for the same reason - listening to how something is being said will detract from what is being said, so grammar and similar are used iun such a way that the majroity of an audience will connect to the output, without getting too far into phraseology and similar.