Taylor Swift did not agree to the contract. Hence why she is withholding her music.

Unlike many other artist, she holds the right to negotiate her streaming contracts directly.

I mean, if she had signed away control of her streaming rights, how would she ever been able to "pull her songs" off of iTunes (or Spotify as she did the previous year).

She would've been in violation of a contract otherwise, and would've need to seek remedy via the courts.