Is the most-shared story always relevant to a wide audience? An organised brigade (think a grassroots political campaign) could use coordinated sharing to bootstrap their way onto the trending list. That would devalue the trending list.
More broadly, you have to teach code to autonomously infer relevance from ambiguous signals. And once you do that, you're still not unbiased; you've simply delegated bias to your audience.
Humans are not perfect, but I would still privilege human editorial judgement above algorithms for the time being. But I agree the question of what issues and perspectives people are exposed to, and who controls what they see, is incredibly important. Just too important to abstract away to a machine.
Also this isn't just about censoring Twitter as a source, but rather twitter as a topic in an trending topic.
Tangentially related: should facebook hide posts made by Twitter employees?
Other than that, it's their platform and their company. They can run it however they want.
Isn't the definition of trending that something got into a positive human-bias feedback loop?
So, Facebook makes an error of using one set of people to decide what is trending for another set of people. And to be honest, I don't think that typical US journalist has the same taste (and the same political preference, for that matter) as general US population.
Does anyone even take Guardian seriously except liberal left socialists living in a dream world?