It goes to the concept of the Fourth Estate - the argument is that the press, as the ears and voice of the people, has an obligation to defend the interests of the people against the other three estates (the clergy, the nobility and the commoners or the executive, the legislative and the judicial depending on who's counting). There's a reason the press in the United States enjoys a lot of freedom - the argument is that a free and functional press is a check on power against the government.

"Editorial" in English means both "proper construction and content" AND "position taken in an argument." The "editorial pages" in a newspaper are where the staff members write opinion pieces, often to augment the reportage of their newspaper. So. A copy editor? Makes the grammar and phrasing better. A content editor? Steers the story or novel or article. An "editorial board?" Decides what formal position the paper takes on political and news issues. So when you see an article saying something like "the libertarian whackadoo has gotten more editorial endorsements than Trump" they're not talking about proofreaders, they're talking about the upper-level people at the paper who issue proclamations, positions and recommendations in their role as member of the fourth estate.