I don't think they're just selling the mastering rights here, it's the rights for the recordings that are being played that is being sold here.

> The new deal with Warner Music Group hands over the rights to the official recordings, meaning the label will profit from any further streaming, radio play or album sales.

Edit: I'm stupid, you mean "master rights", which is correct, they're getting the rights of the masters. Your typo made me think of the act of mastering music, not the "masters".

Fixed - thanks!