Martin, if your question is 'why should adults be treated differently?'; Fred already gave you plenty of reasons why. There's no point trying to twist the argument. The premise here is clear. You are far more likely to be aware of your actions as you grow older and chances are; the opposite is also true. If you are not very aware of the consequences of your actions, then of course an authority should be able to tell you what to do. You can't just expect a child to rationally figure out everything for himself. That's why parents use this method with children; they use corporal punishment to try to implement a sort of behavior which they see appropriate.
So why don't we hit an adult's wrists with a ruler? Because we expect adults to understand the consequences of their actions and be responsive to verbal threats. If you tell an adult, 'don't do this or I will fire you'; he is likely to respond. This isn't the same with kids. At least not all the time. If you tell a child, 'Play nice or I will take away your toys', the child is going to do one of two things. Either he will listen to you, or he will play rough in spite of you.
What happens in schools? You get the kids who had gone through good parenting, and those who had gone through bad parenting. The former group respond well to threats because they understand the consequences of their actions. The latter group does not respond to threats and are likely to rebel against their teachers when presented with one. That's probably why teachers use corporal punishment in some schools, to try to discipline those who constantly misbehave and are unresponsive verbal warnings.
Is that humane? Is it moral? These questions are circumstantial. If a child is causing harm and disruption to his classmates and class and nothing other than corporal punishment can discipline him; then it's not immoral. If a child is spanked and punished and his parents and teachers resort to physical punishment as their first option; then yes, it could be considered immoral.
These methods may seem primitive and unappealing and I don't happen to agree with them personally. I certainly wouldn't want to physically harm my child no matter how little pain is inflicted, but to go out and say that it's child abuse is being a little too dramatic. I see it as a method of parenting that could be effective or not, depending on the child; is it the best method? Probably not. Could it be useful? Maybe. Is it child abuse? Certainly not.