"degrade services that are not paying them" and "allow higher bitrates for services that pay them" would even more blatantly violate FCC net neutrality in the US.

T-Mobile here is circumventing this by only zero-rating with an opt-out mechanism. If they tried to pull a stunt like Deutsche Telekom in Germany, they'd be fined in no time by the FCC in a clear and shut case. They are doing something far more subtle here in the US.

EDIT: For clarity, Deutsche Telekom is a majority shareholder of T-Mobile USA.