Technically, isn't that because anything else would practically impossible and yet we want to have concept like limited jail time and ex-prisoners who live non-criminal lives. In order to allow for such concept, we also allow a less than perfect results from DA's, parole boards, and courts.

If we want to have concept like people being cured of medical conditions, then we must also accept that doctors will make imperfect distinction between healthy and sick.

The question society must make is how far we can accept that mistakes happen, and what safe guards we impose to handle the situations when someone fails.