I don't think people necessarily thought that it was a purely automatic algorithmic creation (although I wouldn't be surprised if that was the case -- remember that most people are not nearly as tech-savvy as the HN crowd in knowing what is or isn't likely to be the result of "pure algorithms"). But the fact that it's called "trending" inherently implies that it's a representative snapshot of what the current attention ecosystem is -- that it's a passive record into what "is trending." By manually inserting or removing things from the "trending" list, that creates a false impression that certain things are or aren't popular.
I'm not saying they are dumb. I'm saying not that many would stop and seriously think if 'trending' news section on FB was possibly being used for spreading propaganda.