Players Seeing Full Map Right Away?

I'm currently watching through Critical Role ( on ep4 right now ), and I noticed something interesting.

It seems that every time they go to a map, Matthew just lays the pre-drawn map down. In one case, he did put down a book to block a portion of the map, but that seems that for the most part the players get to know the layout right away.

When I DM, I've been drawing the map as the players explore -- trying to prevent any metagaming of the "let's go check out that important looking room first" variety.

Is Mercer just super lucky to DM a group of people who can keep their metagaming in check, or is this something that any group can do if the DM ( ie, me ) keeps the players in check?