States have the authority to structure their criminal justice system and ensure the defendant’s role serves the interests of justice. Affirmative defenses, such as self-defense or insanity, introduce new facts that change the legal context of the alleged crime, and legislatures may decide that it’s equitable for defendants to bear this burden since it’s uniquely within their knowledge.
For example: If a person charged with assault claims they acted in self-defense, it is their responsibility to provide evidence such as eyewitness testimony or physical injuries that support this contention.