If T-Mobile wants to pay for something on my behalf, I don't think that's a violation of network neutrality principles for two reasons: First, T-Mobile isn't saying "we pay for your Netflix subscription
and we block access to Hulu." Second, the service is opt-in at the customer's discretion. Giving the customer* a choice is the biggest point of the neutrality argument. If the customer, from an open marketplace, makes the decision, fine. If the service provider, particularly in a natural monopoly position, imposes service-specific limitations on the customer, not fine.
To my mind, T-Mobile's BingeOn (and rate limiting for subscribers who don't have the ONE Plus add-on) is closer to a violation than simply paying for Netflix subscriptions.
* Not that you used this word, but I'm on a mission: I intentionally write "customer" instead of "consumer." We are not consumers; we are customers paying money for a service.