As far as I’m concerned, it is that the punishment is proportional, i.e. the captured runaways have to walk the entire way back. As slaves, they are of course used to going barefoot, so perhaps this can be enhanced by chains. Once back, they receive - publicly - one lash on the bare back per mile they got away, and so on. This will instill a sense of futility, discouraging further runaways (the farther they get, the worse it is for them in the long run). Which is also why it is important that they be caught and returned.