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The game gives you legendaries before the league, in terms of game design this is handing you more tools for victory.
In any other game this isn't nearly as concerning, a legendary weapon in halo isn't condemned by the fanbase for use in the story mode. The player earned it so they get to use it to win.
Its almost as if Pokemon's lore is used to explain why having legendaries be obtainable by the player is somehow cheap. Is it cheap if you walk into the final mission of a game with a rocket launcher that was rather difficult to locate and obtain?