If Facebook truly was unprepared, it’s reasonable to assume they weren’t expecting the announcement.
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.