One TED talk had the quote (roughly paraphrasing) "the media doesn't tell you what to think, it tells you what to think about. That's much more effective".
e.g. the simplest way to manipulate opinion is by being selective in what gets coverage. e.g. if they mentioned UNASUR at all, then disparaged it, then people might disagree and have an input into a conversation about it. By never mentioning it, the US media ensures there's no real conversation about it, because without a shared narrative, it's hard to express an opinion about something. (note this is also Orwell 101, by not having a word for something it's hard to express an opinion about it).
The British press however couldn't give a flying fuck either way so they mention Unasur in headlines when something related to it happens. I'm sure the British have their own shaped narratives that work the same way, except they're about different topics.
Side note: think about how much more effective shaping a narrative by omission was before the internet allowed you to read other nation's newspapers ...
EDIT: I just searched Reuters and even they have articles mentioning UNASUR, they just never get picked up by nytimes or WaPo. even one recent one where Unasur condemned the situation in Venezuela. It appears that even a Venezuela-bashing article that mentions Unasur is Verboten, because not mentioning Unasur is more important than Venezuela-bashing apparently. And everyone knows that even the most tenuous Venezuela-bashing article gets wide coverage. Seriously, UNASUR can't even get a headline in the USA if they all jump up and say "fuck Venezuela" yet Mercosur got one when they did.
BTW: of course the issue is completely not about Unasur per-se, it's just indicative of the general issue of widespread narrative-shaping that goes on in even the most respected corporate journalism.