It is surprising to see that the game does letterboxing. I suppose it's true that the way the game handles multiplayer and secrets in stages, that expanding the camera range wouldn't pan out to "artistic vision" or something like that.
Still, I do question the decision on such a popular title.
And yet, it does make you wonder... How can letterboxes be the only glaring flaw in a title? It's definitely fun. I hate to say it, but I'm eagerly anticipating when people remake older stages, and even completely translate games like SMB3 and SMW to this engine. That will be incredible :)