As for the first question, I say yes. The way I see it: Humans, by default, are about as decent as any wild animal; which is to day, not very. We eventually become trained to act more upright, and respect others. It is a parent's duty to most directly oversee said training. One of the most effective tools in their toolbox is pavlovian training. If a child misbehaves, he'll be punished, and overtime, he'll develop the essential neurological links which make misbehavior seem undesirable. Physical pain is one of the most reliable ways to help form these links.

As for the difference between abuse and spanking is this: Abuse. No really. A good parent would only punish their children to fulfill their duty to produce good citizens. An abusive parent has a more sinister intent; and seeks to harm their children. Now, as abusive parents are a rot unto society, decent government has no particular fondness of them. There's a few tools which they can use to identify and eliminate abusive parenthood. For example, if a child is being bruised badly, or showing obvious psychological damage, the parent either is too irresponsible to raise children, or clearly doesn't have the child's best interests at heart.