That's a fair reason, though you can just teach people to read '() as 'nil' (say in the tutorial).
I'm not second-guessing the decision; I just want to confirm that I'm not missing a more substantial reason. From an implementor's perspective, it's good to know that nil-termination isn't a reason to require a 'compiler pass' for arc. Even if you include 'nil to be eq to '(), ac-denil and ac-niltree are redundant. It seems a strange decision to perform the translation when much of arc is so barebones and no-nonsense.