While here on earth our human body takes all the pleasure in enjoying earthly pleasure and doing sins. Human souls have nothing to do with these pleasures but when the human body dies, it is the soul that is being punished. Isn't it unfair for the soul? Basically, I believe souls are naturally good.
Division of human beings into distinct entities called body and soul is not original biblical (nor generally Semitic) thought but is usually attributed to Greek (and other pagan) philosophies. This is not to say that this division is necessarily false, but there are two points worth mentioning:
- The Old Testament generally relies on the Hebrew belief that man is one, indivisible being. Notions of body and soul come up only in Greek translations and in later translations to modern languages from cultures influenced by Greco-Roman philosophical thought.
- The New Testament dose have some cases of using terms which are translated as body and soul but it has been pointed out that sometimes they convey Hebrew meaning and sometimes not, and this inconsistency can stem from fact that Jesus and the Apostles spoke Aramaic while their teachings were recorded in Greek. Also, their audiences may often have been people from Greco-Roman cultures for whom original Hebrew concepts may not have been understandable.
Now, how this is this relevant to your original question? Well, you call the body "sinful" and in the New Testament we can find references to sinful acts coming from the body, but it is not clear how much of it is just an allegory, a metaphor appropriate to the minds of people from those times, those cultures. One must be careful not to fall into trap of gnosticism, manicheism or other ancient heresies which took some philosophical concepts too seriously and come up with misleading dualisms like holy soul vs sinful body and others. We don't know many things about life after death and all our concepts: body, soul, heaven, hell, are only some simplifications which even visionary mystics describe in various ways.
Also, think about this: even in modern psychology there exists some general notion of dividing human beings into mind and body, but this is a strictly conceptual division which helps us to create useful, ordered perspectives. In reality, every scientist will tell you, that there is no way to separate mind from body, as they're interconnected in so many ways that their separate existence seems completely absurd.
So my answer to your question is: it is not the soul which is punished but a human being regardless of what we ultimately consider him to be in our limited minds.