I saw a mother only a few days ago holding her toddler (looked about 3, perhaps a small 4 or a tall 2) by the arm and walloping him hard on the bum, puntuating every smack with a word "DON'T. HIT. YOUR. BROTHER." Kid looked terrified and furious.
You have to wonder about some people don't you?! And I totally agree with you about what we'd feel if our DP/H 'smacked' us, even if it was gently. Which, by the way, used to be socially acceptable, or at least, tolerated.