It's called corporate partnering, or in some circles, selling out.
Whatever you call it, it's no different that the original iPhone/AT&T deal oh so many years ago. I remember being a VZW customer for years and not being able to get an iPhone because of AT&T exclusivity.
Apparently T-Mobile/Sprint didn't want to play Apple's reindeer games, and so be it. That's their right to do so in the marketplace, same as VZW did with AT&T back in the day, and they regretted it.
T-Mobile is doomed anyway (this is satire, so take it for what it's worth). Poisoned by the malignancy that is the former Nextel. Every corporate entity that merged with or acquired a current or former Nextel partner ended up doomed.
It's like when they open the Ark in Raiders of the Lost Ark. Everyone who keeps their eyes open melts.
The melters are anything that had anything to do with Nextel.....