YouTube – like Google+ – is operated by Google. This however does not stop them from showing only limited compatibility. Here is an example:
I first tried to add the video on Google+ by just pasting the youtube URL and only got an annoying message. This is strange as it used to work until recently. You still can add videos but only in a more time-consuming way.
Update (2011-11-15, 2000 Zulu): The above issue no longer exists. Thank you, Google.