As part of the governance process, Walt Mossberg does bear a certain amount of responsibility for what Facebook management has allowed to happen on their network, and I interpret his action here is part of his taking that responsibility.

What’s wrong with the idea that that he observed something, made a decision, and then acted on it? Or is he presumed to have no agency?

But I'm skeptical - I think the more traditional news media (aka the folks who write stories and collect facts) tends to view news aggregators as a threat to their business model. I think the idea that Facebook ought to be required to screen whatever news it aggregates for "truthfulness" is absurd and I (sometimes) think that the constant refrain of "Facebook did X Bad Thing" is closer to activism than journalism.

But I could be wrong - I'm not as certain as my first post indicated.