I think it stems from the fact that fewer and fewer people are having children at all, and this opinion often comes from people who don't have children, don't understand children, or who have children who walk all over them.
Children are not rational creatures, or at least not always, once you get to a certain age. There are only so many ways to deal with them, and in my opinion, as the father of 4 adult children, spanking or similar correction may be called for in a situation where the child has put himself or someone else into danger.
Because there's one thing people who aren't around children don't understand and that's how many times the average child escapes serious injury or even death. I've seen many situations where no one is being negligent, but which serious, even fatal injuries could easily have happened. You might have the most "enlightened" attitude towards discipline in the world, but that can all change when your child rushes out into traffic or does something else equally dangerous.
Of course, in response to the inescapable dangers of life, we are more and more seeing parents who simply don't allow their kids to be exposed to any source of potential danger, and they grow up to be completely helpless and start voting socialist.