Well do note that youtube did a mass privating of unlisted videos a while back. That might be a factor.
There is no way an unlisted video should be treated as a private video.
And it won't be the last time Google changes opt-out, that's for sure.
Maybe they could have done something more clever, like only private videos that aren't linked on the public web already, but there would still need to be a mass privating of most unlisted videos.