If it's ok to hit a kid then it must be ok to hit an adult as a punishment? If the pain is objective in the punishment, should we slit the skin of the wrong doer with razor blade and drop acid into the wounds? Or is something even more effective in causing pain? Electricy? Medieval torture devices?
I'll address the first part of your question because you make a legit point. I'm just going to dismiss the second point because all of life is a matter of degrees. There are gradations of punishment as there are gradations of everything.
We don't equate adult with children. In our culture children don't have the same rights as adults. We can deprive them things that we don't deprive to adults: certain freedoms, access to products, curfews....
We don't, for the most part, set bed time hours for adults or mandate that they go to school. We don't force adults to eat their vegetables or wear pajamas.
Also there is a big difference between a swat on the hand and a beating. I don't condone beating the crap out of a kid, except in extreme circumstance, but nothing gets their attention faster and better then a smack on the top of the head and instilling in them that outright defiance will lead to a "licking."
And, BTW, I do believe that in certain cases hitting an adult is not only justified but should be encouraged and often a good thing. A friend of mine was sitting with his girlfriend when another guy "playfully" grab one of her boobs. He was quickly floored by a right hook. He apologize and meekly slithered out of the room. I think he will think twice about grabbing boobs without permission in the future. At least if that girl is with a guy who he thinks might kick his ass.
A lot of bad behavior is discourage when the potential offender thinks he might stand a very real chance of getting his ass kicked.
