Purex,
Any form of punishment is abusive if it is done to an abusive degree. There will be people who abuse grounding, people who abuse withheld privledges, and people who abuse corporal punishment. I knew a kid who was grounded for a month for being home 5 minutes after his curfew on a weekend. To me, that is abusive.
I have experience with abuse. My parents both used corporal punishment. My mother was abusive, my father was not.
I can count on the fingers of my hands the times my father spanked me. Each time, it was for doing (or not doing) something after he made his rules and instructions and expectations crystal clear. And with the punishment, both before and after, came the lecture. It worked. My father had 2 hard-headed kids, my one brother and myself, and we required this. The other 2 might have been spanked 2 or 3 times. All he had to do was look at them and they would stop whatever misbehavior they were doing. And every one of his punishments was not corporal--he made the punishment fit the crime.
My mother, on the other hand, beat kids because she had a bad day. Or because she was angry with someone. Or when we did something that warranted punishment, she went over the top. She hit my sister in the head with an iron because she hadn't done the dishes as quickly as my mother wanted. She threw knives at my brother and I because we walked through the house with muddy shoes. My dad's reaction to that would have been get out the mop and broom and clean not only where you tracked but the rest of that room as well. I remember her throwing a 50# sack of dogfood on my sister for being on the phone too long. My sister was about 90# at the time. She also pushed me toward a fire when we were burning brush--the fire was low enough that I jumped over it. I could have just as easily fallen into it. My mother was also a hair puller, and don't let her get ahold of your hands--I have had fingers broken and dislocated for not being quick enough.
Let me take the case of the original kid who was grounded for getting home 5 minutes late. If I had done that, my father would have told me to be more careful the next time. 5 minutes--it's obvious that the kid was trying to get home on time--it's not a couple of hours. There would have been no punishment for that unless I kept getting a little later each time, seeing how far I could push it. For that, I would have spent a couple of nights grounded.
And for my father, corporal punishment was a punishment for a certain age. By the time we got 10 or so, it wasn't used. I think my brother was 12 the last time my father used it--he got the belt for throwing rotten squashes at a neighbor's passing truck and causing the guy to run off the road. Thankfully the man was not hurt nor his truck harmed, but my brother had to wash, wax, and buff the entire truck and personally apologize to the guy, and he got the lecture in addition as well. And he had to spend a day helping that neighbor dig postholes without pay.
The incident was not repeated.
These are the things that I consider to be abusive punishment.
1) When punishment is unwarranted.
2) When the person punished did not know that what they did or did not do was wrong.
3) When the form of punishment is excessive for the infraction.
4) When punishment is not followed up by addressing the issue of the infraction.
5) Broken bones, burns, sprains, cuts. (I don't include welts as excessive)
6) When the amount of punishment is excessive. 5 or 6 swats is not excessive. 40 is over the top. Grounding a kid for a month for being 5 minutes late is downright draconian.