It makes it harder for other businesses to enter the same market, as their offerings will not be zero-rated without an agreement with T-Mobile.
Any measure that makes it more difficult for companies to complete will ultimately harm consumers in the long run. So the only people this benefits, ultimately, are the incumbent providers, because it makes their market less competitive. Startups, for one, definitely lose: http://avc.com/2014/01/vc-pitches-in-a-year-or-two/
Please specify one startup that this directly impacts. Which streaming service is left out in the cold, and in what way does this specific situation stifle competition?
Data caps on cellular data plans already exist, so getting uncapped data for specific services does only benefit the consumer.
Your generic "net neutrality is good" arguments don't apply here, where net neutrality has been broken for years. Net neutrality hasn't ever existed on the web, let alone cellular data plans, anyway.
It's in the same vein as, "unlimited in-network calls".
I just feel like folks don't realize the fact that net neutrality has never purely existed, not since peering agreements were first established.