Telling the child not to open store merchandise is reasonable parenting. Beyond that, what this man said to the child was abusive. Unfortunately, there is nothing that can be done about it by the authorities, and I don't think etiquette allows people to step in on the child's behalf in a situation like this.
I think abusive is a really strong word to use here especially since we have no idea of exactly what happened. I can honestly say that I have been told more then once by my parents that I am a pain in the bacon-fed knave. I lived to tell the tale. I dont feel abused or mistreated~as a matter of fact I was being a pain in the bacon-fed knave!
As far as the old "Ill give you something to cry about" cliche, I mean come on. Its just a saying. Perhaps not the nicest saying but not abusive~more laughable then abusive really since I have never seen a parent actually follow thru.