Really looking forward to this. Nothing else needs to be said, really.
As someone said, it's because the album art it references is square. I agree it's a shame, because for this particular cover they could've just extended the white to fill the cover and it would have looked better. But they're giving all the hip-hop variants a uniform look and for some of the others it's obvious that you can't fill the cover:
I have the original painting they made the cover from, though, and THAT is 100% awesome. :D