“So, if editorial boards wouldn’t even imagine endorsing any candidate/party, it can’t be because they think it’s completely wrong, that it’s anti the role of a news provider?”
Editorial boards are free to decide on their own what they think about endorsing candidates (note: editorial boards don’t endorse parties as a whole). If they think it’s wrong, so be it.
“And how about just opinions from the public (columns by random Joe /Jill public, and/or letters to the editor?”
Those get published all the time. That’s what the opinion section is for (and opinion pages frequently publish both letters and op-eds that express opinions contrary to their editorial board’s endorsements). Being informed requires more than just a mindless recitation of contextless facts. It requires thinking about the implications of those facts, which in turn requires inquiry and debate.
“The fact that it’s the editorial board makes it the newspaper endorsing it doesn’t it?”
No, because a newspaper is more than just the editorial board. The editorial board is the group that curates the opinion section of the newspaper.
“That’s how it’s reported Newspaper x endorses…”
It’s called “shorthand” or “metonymy.” When people say “the White House claims…” you don’t think it’s the actual building talking, right?
“By the way, my OP says part of, not the mere existence.”
And so does my answer, which you would know if you had bothered to do more than just scan it for something to disagree with: “so if the mere existence of opinion writers, editorial boards, and endorsements is part of why Trump calls the mainstream media ‘fake news’ (a term that mostly referred to pro-Trump propaganda stories before he reappropriated it as ‘anything I don’t like’), then it’s just one more reason to think that he is the actual enemy of the people.”
@Pinguidchance That’s why it’s a newspaper’s editorial board, and not its journalists, who write the endorsements.