Usually people from the press or beta testers have early access to games. In these cases the publishers prevent any leaked info via NDA's (non-disclosure agreements) that have to be signed by the ones with the early access. This prohibits anyone who signed the NDA from streaming the game and if caught, will result in takedown notices by the publisher, a takedown of the stream, the deletion of any material (recordings/videos) on Smashcast that include the game and in the worst case a suspension of your channel.
Sometimes retailers send out/sell games before their official release date. In these cases there is no NDA in place to prevent you from streaming the game. Still, publishers usually don't want the content of their games being leaked before the official release date and can issue takedown notices on anyone caught streaming the game, again resulting in a takedown of the stream, deletion of any material and in the worst case a suspension of your channel.
In any case, we strongly advise against streaming games before their official release-date. Not just to prevent your stream from being taken down and your channel from being suspended, but also to not spoil viewers who want to pick up the game on the official release date.