Almost all video streaming services will downgrade the stream quality if they get a 1.5Mbps pipe, and any that don't are at a competitive disadvantage to anyone who is streaming from a phone and who actually only has a 1.5Mbps pipe. It's built into most of the streaming protocols to downgrade on a slower connection. Source: I worked with video streaming on the client side when I was at Amazon, and additionally researched other options to the one we settled on.
So T-Mobile is asking users to choose between a (potentially) lower quality video stream in exchange for getting tons of video streaming options for free. This reduces load on their network, because people were obviously streaming anyway, and has convinced the biggest providers like Netflix to voluntarily reduce their bandwidth consumption to 480p in order to get free streaming to T-Mobile customers.
At best I would say that better user education could be called for, or potentially forcing the users to make an opt-in/out decision, but honestly I think the users would hate the interruption. This is clearly a situation like the European cookie permission law: If you force them to ask the users whether it's OK, sure they will ask, and that will interfere with the UX and piss off users.
I'm fine with it being opt-out; let T-Mobile email or send a notification to everyone about how to opt out if they want, along with an explanation of the consequences, and let them decide whether to click the "opt out" button.
Not "because T-Mobile," like another comment implied, but because it's a fine trade-off, and anyone who cares can just disable it. More than 480p on a tiny screen is nice sometimes, but I'll take free 480p over the cell network and watch my 1080p video when I'm at home on wifi.
I also love to be able to stream music for free on T-Mobile. That's a feature I use all the time. Even my more "fringe" music streaming services are supported. This doesn't seem to me to be anti-competitive, and T-Mobile isn't charging anything to stream the music, so I think it's pure awesome.