If they're fighting holograms, which they are, why wouldn't the holograms look like and act like Covenant? Spartans don't fight the same as Covenant, so learning your fellow Spartans' tactics won't really prepare you for the Covenant's tactics. As for the American soldiers training against other American soldiers, that would work much better since Russian soldiers tactics are still very similar since they're both human. We're talking about an alien race they would be fighting in these War Games.