In my experience typical justifications for beating the kids turn out to be flimsy excuses pulled out from scriptures (Hadeeth regarding prayers) and out of context as explained below.
A parent typically beats the kid when he is angry and not in his senses. When the parent is irritated, frustrated, or feeling weak due to some illness, or may be angry due to fight with spouse/boss can make the otherwise reasonable parent vent his/her frustration and beat the kid.
Parents do not beat the kid when they are cool minded or in their senses. They beat their kids when their egos get hurt which happens when children refuse to obey their parents. The cause is not the violation of rule but the violation of ego!