It might not make them criminals, but it does make them bullies. It might not be abuse but it is violence.
If people are happy to raise children using bullying tactics that are legal, then that is their business, but at least acknowledge that they prefer to use fear, threats of violence, intimidation and pain to encourage the behaviour they want.
As far as I'm concerned it is an abuse of power and responsibility which is not tolerated in any other setting or relationship, and just because you can get away with it, does not make it right.
There was a time it was OK to slap your wife if she behaved in a way that was unacceptable to her husband. Yes she might have been an adult, but she was also normally financially dependant on him, smaller than him, weaker than him and very vulnerable-not unlike a child. Thankfully we have moved on, but not nearly enough in my opinion.
I will never understand the argument, "I use a light tap." If it was such a light tap then it would be pointless. It is surely enough force to either scare them or hurt them, otherwise why would you bother? If my elderly, confused relative ran into a road, I wouldn't slap them to teach them a lesson, I would accept that they needed closer supervision and possibly restraining.
Plenty of people live their lives without resorting to violence. It is possible to raise well behaved children without scaring them or hurting them, but I suppose sometimes it is easier to just "giv 'em a whack".