[vore]

It is not fair use in itself: https://courses.cs.duke.edu/cps182s/fall02/cscopyright/Copyr...

  Decompiling object code produces an approximation of the original source code. Merely making this rough copy would usually violate the copyright holder's exclusive rights, even if the person who decompiled the code only used it as a preliminary step in making another work.

The only reason anyone believes this is fair use is because the copyright holders have chosen not to enforce it.

[EMIRELADERO]

It is more likely to be fair use in this specific instance, as the copyrighted work to which the analysis is applied to is the whole game, not the code itself.

A big part of fair use analysis is in the effect on the market for the original work. If people have to obtain a copy to even use the project in any interactable way, the effect is either none (if we suppose every user of the project gets their original copy illegaly) or positive/benefitial to the copyright holder (if it makes those users actually buy legit copies)

[charcircuit]

The game is a copyrighted work, but so is the source code. The compiled code is a derivative work, and so is the decompiled code. Nintendo is not selling the source code, so arguing about sales in the market does not makes sense.

[EMIRELADERO]

The fair use analysis is done on the work as a whole, the Zelda game that's registered on the Copyright Office. Not the code only.