I'd venture that they're extremely wealthy, still friends, and decided it's time to go have fun rather than prescribe this to Facebook's ethical challenges like the article would have you believe.
edit: fixed punctuation. By "prepared explanation" I don't mean a formal press release, but a reason (even if vague) that can be publicly disclosed, even before the official announcement, so it doesn't look like the founders are jumping ship for bad/antagonistic reasons.
Because people worked under and followed the for the better years of their life to make them rich?
Because the people you work with are more important than the work you do?
Because when you form a relationship with someone, contractually, business, or otherwise, you do, in fact owe them something, be it money, respect, or even some simple common decency.
If Facebook truly was unprepared, it’s reasonable to assume they weren’t expecting the announcement.
My point was that it was quite a stretch for the Times to equate this situation with Jan Koum's departure and this narrative around Facebook's moral dilemma. These guys stuck around for nearly a decade, which is frankly the story that should be written about given how infrequently that happens.
I agree with your point though.