Inherent impossibility

In the journal that I mentioned in the last thread the author mentions an interesting legal concept that I'd never heard of before: inherent impossibility.

In general when a person makes an unsuccessful attempt to kill somebody, such as trying to put poison into somebody's food and by accident putting salt instead, or shooting a corpse while believing that the corpse is a live human being, the person is convicted of an attempted murder. Inherent impossibility is a defence for the cases where the method chosen by the defendant is inherently impossible from the point of view of a reasonable person: for example making a voodoo doll of a person and sticking pins in it would not be an attempted murder no matter how much the perpetrator believed it would kill the person, because of the inherent impossibility that such action would actually kill somebody.

Intent and planning is an interesting topic in criminal law. The laws of most (all?) Western countries tend to punish a planned crime harsher than a similar crime made impulsively. Is it because people who have reasonably good impulse control have a higher responsibility to society than the ones who don't? Is it because people who plan their crimes are considered a higher menace to society than ones who just "can't control their urges"? Is it just plain discrimination against the kind of people who plan everything they do, including murder?