If there's a guy who, say, committed a robbery in your neighborhood, physical punishment may or may not deter him from future robberies. You don't know and in the meantime he's still around. But if that guy gets sent to prison, the state guarantees that he will not be around for a fairly long time.

This is totally obvious, I'm not sure why you felt you needed to point that out.

The point of my comment is that it is interesting that prison isn't viewed as cruel, even though it's obviously more harsh than alternatives. Obviously there are other reasons people prefer prison as a punishment for others.