It's well known (I've read articles and heard from inside sources) that service providers like FB and Twitter partner up with big carriers to make use of their app free for carrier customers. It's possible that these music providers have set up similar deals. But could also be that T-Mobile is creative with their offerings (which they're known for) and offer free music an USP against others like AT&T.

I'm torn. As a T-Mobile customer, it definitely benefits me. As a net neutrality advocate, it does concern me.