This is definitely one of those issues that's NOT black and white. It depends on the child, the age, the circumstances, the parents ability to refrain from spanking out of anger... so many things are part of the equation. I do not believe spanking (a swat that does not leave a mark) is child abuse. But I also believe that spanking should be a last resort and not the first thing a parent does when their child acts out.
I just don't think any parent should be criticized for how they choose to discipline their own child that they know and love better than anyone.