As long as you toe the line
The entire decisionHere's the full decision, in Dutch, as published by ACM is rather long and juridicalAnd here's a translation of the key paragraphs of the decision. The ACM states in its decision that T-Mobile does not discriminate against music services, “as long as they comply with the conditions that T-Mobile imposes”. Well, duh, that's exactly the problem! T-Mobile is imposing all kinds of conditions upon these music services and therefore decides which ones are entitled to preferential treatment. For example, if a music service doesn't want T-Mobile to use its logo, they're out of luck. If the music service isn't able or willing to structure its systems to the whims of T-Mobile, they're left out in the cold.
The ACM also thinks that your freedom or the freedom of the music service provider isn't being constrained. That's hardly surprising when you realize that the ACM seems to have no regard for the impact of services like T-Mobile's Data-free Music on the innovative nature of the internet. One of the powerful properties of the internet is precisely that every computer, and therefore every service and user, is as easy to reach as any other. A service like T-Mobile's flies in the face of that idea by giving some services preferential treatment.
The ACM (judging from the text of the decision) also hasn't taken into account what other operators will do if this is allowed and how that will impact the freedoms of internet users.