As reported by Engadget
Universal and Sony Prohibit Zune Sharing for Certain Artists
In a non-scientific sampling of popular artists by Zunerama and Zune Thoughts, it looks like it’s roughly 40-50 percent of artists that fall under this prohibited banner, and the worst news is that there’s no warning that a song might be unsharable until you actually try to send it and fail.
Wel-come to the social. Indeed!