^ Wasn't Rihanna's album seen as R&B? I remember when it dropped people were saying she wouldn't have as many hits because so many of the songs were urban.
Justin may be pop overall, but I think "Pusher Love Girl" is an urban song with that heavy neo-soul influence. It also has a lot of funk in it. He didn't win R&B Artist, he won for a specific song.
What case could be made for M&L being pop?