Well, the main reason the new Call of Duty Warzone mode is free to play is due to the opportunities for the players and the companies who manage the mode. As so, this is recurrent through other free-to-play games, such as, Fortnite, Call of Duty BO4 Blackout, and others on Steam, and elsewhere.

the campaign is not free, the game consists of warzone, campaign and other, only warzone is free to play, you need to purchase the full game to access all the other content.