Youtube doesn't use AllShare (DLNA) to share to Samsung TV because that's will break some laws (license maybe) you maybe see also when using Screen Mirroring/AllShare Cast/Miracast that some VOD application won't mirrored to TV.
Youtube using DIAL as the share protocol with TV and it makes the TV to launch the Youtube installed application in the TV to play selected videos, so the streaming will be done by the TV, with this mechanism there will be no issue in media license for mobile and TV.