I also don't like the way some parents are legislated, but...
Corporal punishment has been proven to be ineffective and counter-productive:
http://www.comm.umn.edu/~akoerner/cours ... 002%29.pdf
Not only that, but it is a barbaric practice that reeks of substandard intelligence on the part of the parent.
It is simple common sense that the human brain is most open to learning when a human is engaged and secure. The infliction of pain triggers a subconscious panic response which interrupts learning function in the prefrontal cortex of our brain (where social learning occurs).
When I see a parent hitting their child, I often wonder if they realize they are actually cementing and deepening the maladjustment or misinformation in the social programming in their child's brain. If you think your child is behaving poorly, the worst response is to inflict pain. Quite literally. While people often laugh at this, it is vastly more effective to immediately engage the child in a non-physical way and attempt to reprogram the behavior directly, in a way that engages their inquisitive mind. Ask them questions. Demand answers. Turn it into a test. With a reward for good performance and neutral cool response for negative.
I'm not pulling that from my ass. There is a significant body of scientific evidence to establish such theories. Further, one can witness the success of these systems in the training of dogs and monkeys, both of which are also mammalian, and therefore share similar characteristics with humans in the development of the prefontal cortex.
Alas, it will take some time to reprogram the adults, though!