Oh look! They're doing exactly that with exclusives.
As a listener you have a choice of pay now for access or wait until it's streaming everywhere else.
Spotify, so far, has been losing money. Overall, monetizing music is just a very hard business. I feel that point is often missed by artists who want to put the blame on Spotify as if Spotify is sitting on billions in profits that could go to the artists. Apple temporarily enabling a few artists to make some extra money doesn't change the fact that music is a very hard business to make money in--as much for the broadcasters like Spotify as for the artist.