That's a good question. Philosophically, I think of a player's agency in the game as kind of ghost like. When you're doing Chief stuff, being a pretty cool guy, doesn't afraiding of anything, then that's just first person agency in the game universe. But when you're reading. Terminal in anniversary, you're a ghost, haunting the game, and observing things nobody is expected to see. The Anniversary terminals give players insight into the nature of 343Guilty Spark's madness and occasionally revealing subtext and context for some of his crazier statements. The chief and even cortana, wouldn't know these things.
One of my favorite moments at TGS was watching some of the real halo fans from the press, laugh at the exact same moment in the first terminal, where it becomes apparent that Spark really does just want to help, as long as you listen to his instructions. And that's something you're definitely not 'supposed' to know.
