Children are driven mainly by emotion. Particularly young children. Pain is an emotion that tends to override other emotions. A brief, sharp pain can snap them out of an emotional loop they found themselves caught in. Since the pain is very brief, they tend to revert back to a much more reasonable state afterwards.

All that said, yes there is a difference between parenting and abuse. However, children are not tiny adults. They are tiny humans. You can often tell how they are feeling or why they are feeling that way, but it isn't by thinking the way a well-adjusted adult would. The whole point of parenting is to help them get to that point eventually.