Comcast is probably going after a kickback from the device creators. Apple (iOS), Google (Android), Microsoft (Xbox, Tablets) have no problem with this. Roku is a much smaller company and probably doesn't have many carriers that won't do it for free. Same reason why Samsung TVs don't have access, because Samsung won't play ball with Comcast directly.
As for the game consoles, money changed hands between HBO and Microsoft to keep it exclusive to the Xbox platform.