http://www.androidpolice.com/2013/11/27/apk-teardown-youtube...
i.e. By enabling the Android/iOS Youtube app to background (which they've gone out of their way to disable up until this point), it essentially becomes an audio streaming service with the same interface.
Plus according to the FT, the problem the indie labels have isn't with the subscription rate per se, its how the new ad tier is set up:
"One label boss said the big problem with YouTube’s new licensing agreement was not to do with the paid tier, but rather that it allowed YouTube to make substantial enhancements to its free tier. His fear is that YouTube’s free tier will become so attractive that it will reduce the number of people willing to pay for subscription services such as Spotify or Deezer."