Yes, it is an affirmative defense by method of determination. But I just don't like the stigma that goes along with the term "affirmative defense". That would make Fair Use viewed as a less admirable thing like most affirmative defenses. I had a debate with someone on Reddit about this and the other person claimed it was an affirmative defense like self defense is for murder. In that case, you still committed murder and it was still illegal, but in that special case, it was ruled as a permissible illegal act. I argued that Fair Use was unlike that because a use that is Fair Use is not an infringement of copyright.
Do we disagree on any of that?
Eh, I think that part of the stigma of affirmative defenses just occurs because of extra-legal gut reactions in criminal cases. So I'm not sure if that should change our determination here (firstly because it's extra-legal, but secondly because copyright infringement is generally not a criminal charge.)
But if a person is tried for murder (a particular form of criminal homicide) and their self defense claim is successful, then it's not true that they "still committed murder and it was still illegal, but in that special case, it was ruled as a permissible illegal act." Homicide is not always an illegal act. To the contrary, only criminal homicides (of which murder is a specific type of criminal homicide) are illegal acts, but a determination of self-defense means that someone didn't commit murder. While they still have committed homicide, it's justifiable homicide, which is not illegal (or even a "permissible illegal act.")
I mean, self-defense is an inherent right (see Supreme Court in
Heller.) Ultimately, the Supreme Court thinks that self-defense is a right via the 2nd Amendment as much as free speech is a right via 1st Amendment. So self-defense acts are definitely lawful actions (even if yes, they result in homicide). The affirmative defense part comes into play in that if you are in a situation like this, you'll never get to that outcome unless you have a court decide it.
Here, the fact that a homicide occurred is not in dispute. In the case of using self-defense as an affirmative defense against a murder charge, one is not even disputing things such as malice aforethought. (So, it's not a negative defense that challenges the original claim that "he killed someone and had malice aforethought") Rather, self-defense is the introduction of additional facts that when accepted defeat the murder charge. "Yes, he killed someone and had malice aforethought, but that intention was in self defense, so it's not murder."
I think that's similar in copyright infringement and fair use.
No one is disputing that the copyrighted material was used without permission. So there isn't a dispute of existing facts that would, without additional facts, support copyright infringement. To the contrary, one asserts additional facts that defeat the copyright infringement charge. "He used copyrighted material without permission...but he [transformed the work/didn't use for commercial purposes/didn't use a substantial portion of the work/etc.,]"