Here is the thing - a law that doesn't get used is not really that useful. If we look at countries where it is against the law, you don't see that it is really used a deterrent in cases where it is just smacking. One of the basic principles of good lawmaking is that you only make laws you can and intend to enforce. They aren't just signals of what is socially ok or not.
It's already illegal to physically abuse or beat a child. And yes, there is a difference between a smack that is not intended to cause harm, and a beating that is.
We could also ask what a law like this could lead to. A few people have mentioned that they found being yelled at more stressful than being smacked. If you look at groups that are very anti-smacking, many go on to say, and these other punishments are also equally stressful - time out, taking away toys or privileges, yelling, etc. Whether or not these are very good methods, if we can show they could potentially cause as much harm as smacking to a child, are we comfortable wit the idea they also might be made illegal.
What is the potential for, say, an emotionally abusive parent to accuse another parent of smacking in order to cut off access?
The other thing I'd say is the research on the effects of smacking is pretty ambivalent. It seems to suggest that it's mainly damaging to kids when it's not common, in societies where it is common kids aren't so negatively affected by it.
Laws should really as much as possible be based on evidence.