T-Mobile is the third-largest carrier in the US. Probably a small minority of T-Mobile users even know about the free streaming feature. If your business plan requires that tiny market segment to bootstrap you, then you need a new business plan.
I just don't see it as a big enough differentiator to kill a new service, even if other carriers started doing something similar to T-Mobile.
A new streaming service doesn't need to rely on free mobile streaming at all to bootstrap; if its focus were mobile streaming, a better answer could be to pre-cache hours worth of streams when on wifi. I end up in dead areas frequently, and I end up in roaming areas where T-Mobile won't give me data at all. I'd love a service that would auto-cache exactly the music I like to listen to so I could play it when roaming.
I've heard Spotify CAN cache, but it's too expensive for my tastes. Something like a cacheable Pandora would be nice.
But my point is that while yes, it grants an edge to certain established businesses, it's not an insurmountable barrier. Not like Comcast throttling Netflix; if you can't use the service in its primary form, it's DOA. And T-Mobile seems to be adding new services pretty quickly: Even indie stations like Radio Paradise are on the list.