I have two boys, both very young, and have been to playgroups where older children often behave quite violently towards other children. These children seem to have no idea of the hurt they are causing.
If a child is smacked (once they've been told it's going to happen if they don't change their behaviour and obviously not hard enough to cause a bruise) then they get the idea that physical pain is a punishment. If they don't know what it feels like how do they know what they are doing to other people? It seems to come naturally to children to try hitting as a way of expressing frustration at an age when they're not capable of verbal reasoning, so an adult giving them a smack to control their behaviour at that age wouldn't seem unreasonable.
I believe that most children start out basically nice and, when they get to the stage where they understand other people have feelings too, they won't want to inflict something they see as a punishment without due cause.
I know this message is very nonPC but I also know some people agree, although to actually smack a child in public runs the high risk of abuse from other people. Can the choice to smack or not be accepted as a parenting choice without interference from other people?