Your name reflects what you're views are: neglectful. Are you serious? I've gotten much worse than a slap to the ass. You ever been hit with a belt, or a switch? My mom hit me with a shoe once (sandals, I think) just because she couldn't get to a belt. It works out for the best for children, it really is one of the best ways to teach them not to do something again. I'm not saying it's the right solution for everything that kids do wrong, but to call the guy an imbecile for it....well, the guy is more than an imbecile considering why he spanked the kid. But the trick is to distinguish that line between discipline and abuse.
As a child and into my early teens, I did experience child abuse. Is it neglectful to call someone on their bullshit? All people are doing are directing your eyes towards the word's connotation: 'spanking'. It's irrelevant what you call it, because at the end of the day you're still beating your child.
I don't see why violence always has to be a resolution. This single-minded mentality is absurd, and you're an asshole to assert any sort of justification by labeling it off as "discipline".