It appears that NBA TV is already in streaming media.
https://en.m.wikipedia.org/wiki/NBA_TV
You can view it through Sling TV, PS, Youtube, fubo TV, Direct TV now. Here is a blog discussing a multitude of options for watching NBA on Roku.
https://blog.roku.com/stream-nba-games-roku
From NBA or any sports league POV they would want the game seen from as many locations as possible. Roku may very well be the dominant smart TV OS 10 years down. But it would still have to offer many options for watching your favorite action or shows for that matter. Many of the above options - Roku cannot make a lot of money on unlike smaller channels like Crackle etc. That being the case why should having the dominant OS translate to the highest CTV advert market share?