THIS is what we were afraid of when the ISPs started to break network neutrality! So now, T-Mobile will "give" you "unlimited" music...IF you use the apps that they support. Use a different music service? Too bad...you get charged more money! Or they throttle the speeds to the point where streaming is unlistenable.
This is the WORST thing to happen in networking in a very long time. It means that T-Mobile gets to extract extra money from the big players...and if they don't pay...YOU do! It means that all data is not being treated equally....you get charged more for listening to one app than you do listening from another!
Next you will see the same thing applied to video streaming as well. "Unlimited Netflix"...but not YouTube or Hulu..."Unlimited Vudu" but not Google Play Movies or Redbox Instant.
I highly suggest calling T-Mobile, if they are your provider, and INSISTING that they stop this practice IMMEDIATELY! If they do not, I would highly recommend moving to a different cell provider!