What matters for procedure is, what's reasonable. It's reasonable that someone who claims they killed in self-defense would have to present evidence that they felt they were in serious danger. It's reasonable that someone who claims they were not sane at the time they committed a robbery, should have to explain with evidence the causes and circumstances of their insanity that distinguish their mental state from other mental states that we'd treat normally, like that of simply being greedy. These are routine situations where the burden of proof is on the defendant to mount the corresponding "affirmative defense." They arise because of a more fundamental principle that if you did fail to obey the law, but you want to argue that it's OK that you failed to do so, then you should be able to tell the court why you failed to do so, so that we can indeed evaluate whether it's OK.
The only difference here is that insanity, self-defense, entrapment and the like -- traditional affirmative defenses -- are self-evidently very exceptional circumstances. Forgetting is, by comparison, quite routine. Therefore the courts are actually incredibly lenient, putting only a weak burden of production on you so that the question of "why can't you?" is answered up-front, and then this reason will be evaluated under the assumption that you're innocent until proven guilty.