That private-thing is new? I never saw this before.
Yeah, it's fairly new. The option has probably only been available for a year or so, if even that. Thing is though, there are probably a lot of YouTube users (and just people online in general) that don't even know of it's existence. After all, it doesn't really make a lot of sense to upload a video to YouTube, only to then make it private (which thus makes it unable to be viewed by other people).
It's totally up to the individual uploader though, so it's not like there's a lot that viewers can do if they find and try to watch a video, only to find out that they can't because it's been marked private.