I'm not particularly sure what good making Pistorius stay in prison would do. We put people in prison to physically stop them from reoffending, to give them a punishment that makes them think twice about reoffending in the future, to scare others into deciding not to commit the same crime, and to put them in an environment where they can learn skills and attitudes that will encourage them to be law-abiding when they get out. None of those apply here. Unless you want to throw out the verdict of the court (which is neither here nor there: whatever you think of it, you cannot expect the South African legal system to proceed on the assumption that its own sentence was undeserved!), none of those apply here: nobody (by definition) is contemplating accidentally killing their girlfriend by mistaking them for a robber, and Pistorius is if anything less likely than your average man on the street to commit such a crime again.
So, why put him in prison? We use prison as a kind of metaphor - lots of people in prison means we're being 'tough on crime', and that's a Good Thing. When they're in prison, they're supposed to Suffer, because bad people suffering is a good thing. Whether it is actually achieving anything for society, like (say) reducing the number of people committing crimes, is hardly ever mentioned.
So, why put him in prison? We use prison as a kind of metaphor - lots of people in prison means we're being 'tough on crime', and that's a Good Thing. When they're in prison, they're supposed to Suffer, because bad people suffering is a good thing. Whether it is actually achieving anything for society, like (say) reducing the number of people committing crimes, is hardly ever mentioned.