There are also beasts called editors and subeditors, whose jobs includes watching for editorialising and stopping it happening, checking facts and maintaining standards. News journalism isn't blogging or writing stuff on forums.
Also, many news journalists also write opinion pieces, blogs, columns, analyses and other kinds of articles where an opinion is appropriate, so they're aware of the difference. The only personal opinion that is appropriate -- in fact, essential -- in news is that "This is worth telling the readers about". Yes, that does include things that to the experts is no news at all.
Such distinctions matter and are observed, often with some vigour.
Also, journalists hang around places with people who know what's going on, and do try to learn stuff that helps them do their job better - amazingly, I know, some hacks take pride in their work, and not just because everything they do appears in public with their name on it and, these days, is Googleable forever. (How many Ppruners would be comfortable with that as a working practice?).
This includes places like here, where one does tend to be told on a regular basis that one's profession is entirely composed of dangerously incompetent twerps with loaded keyboards and no sense of aim, and it'd be far better if one just went away and left the real men to it. That's usually a good sign that it's worth sticking around!
R