I believe that was the unofficial compromise between despotify developers and spotify. Since despotify devs pointed out initially on their page that spotify could track and stop it, but then despotify would just put out an update that would make it virtually untrackable. (which is why they didn't bother masking the client to begin with) So Spotify said, ok we'll let you do that as long as the people who use it are premium members, so despotify changed their program to require premium membership and everyone is happy.

Probably the only reason why it's not official is that music companies wouldn't like it.