A couple of reasons. One is that your paperwork guarantees you'll pay attention to their "throttle this down to sub-480p" request, which let them have their customers (who opted into it from t-mobile) not have to worry about how that apps streaming was free. That is, T-Mobile customers who turned that feature on could be presented whitelisted apps without needing to configure them. Two is that "free video streaming" sounds better when selling a service to their customers than "free Xmbps connections". Three is it allows a lot more freedom to T-mobile. They know they need to average X mbps so the stream doesn't jitter, but they can totally take advantage of an uncongested five minutes to let you download quickly to build your cache.