The Verge recently laid out in detail an old leaked contract between Spotify and some label.
They all got little important pieces in these contracts that say "if anybody gets a better deal, we get it too." So by Apple agreeing to pay some for the trial they probably have to pay them all. The already signed contracts have already made sure of it.
But they can afford it and they really should pay for their own promotions, they want to lure customers so it is their duty to pay.
I don't buy it because the math doesn't work out. Apple has almost a billion customers with credit cards attached, let's be generous and say 40% of them try out Apple Music, $7 per customer goes to the labels per three months so that should be $21 per customer. (21)(400,000,000) = 8.4 billion dollars. Are you telling me Apple is willing to throw out 8.4 billion dollars just because Siwft's rant? Obviously that's not gonna happen.