Writing opinion and reporting on events are different things. Opinions are usually denoted at masthead, preamble and postscript.

Why do you think that readers don’t understand that?

Opinions aren't meant to be neutral and fair, and it isn't a violation of journalistic ethics to publish them as such.

If they had tried to disguise opinions as journalism by introducing intentional bias and distortion into a story, then that would be a problem. But newspapers have published opinions for ages.

It's rather naive to think that newspapers ought to be neutral (or fair) in everything they produce. What kinds of neutral is desirable? There's neutral tone or neutral political bias -- there are many different ways for a newspaper to be or not be neutral.

Assuming neutrality isn't something that we should expect newspapers to value, then I think transparency is an good alternative. A presidential endorsement can be a good thing in that the newspaper staff are being openly transparent about their political bias.