I think that's because we don't actually smack them hard enough to 'hurt' them. And I think the tears are more out of fright than anything kwim?
True! We've got this game called "penny a smack" where we smack dd5's bottom in play, quite hard sometimes too, and she thinks it's hysterically funny! I've often said to her, if we did that to you and we were cross, you'd be crying!!