The Copyright Office factsheet on games explains exactly this: Copyright does not protect the idea for game, its name or title, or the method or methods for playing it. Nor does copyright protect any idea, system, method, device, or trademark material involved in developing, merchandising, or playing a game.
In these cases, games that are "clones" of another are generally not implied to have committed any intellectual property infractions, and otherwise considered legally acceptable practices, although calling such games clones is generally considered derogatory.
Many believe Call of Duty: Black Ops 2 provides one of the best experiences that the storied fist-person shooter franchise has to offer. Considering its unique branching single-player, perfectly balanced multiplayer, and addictive zombies mode, the title had something big to offer for a huge array of fans.
What is your review of Call of Duty: Black Ops 3? I would give it a solid 7.5/10. It is not a great game, it will not go down in history, but it is very fun, well-designed, and really WORKS mechanically, which most Call of Duty games have not done.