I do agree that the methods that many parents employ to try to attain the worthy goals are potentially as damaging as smacking.
Encouraging children to eat their vegetables is a million miles from standing over them for hours and angrily forcing them to eat foods that they find intolerable.
Personally, I remain to be convinced about grounding as an acceptable form of punishment.
I think there is an important balance between what a child is old enough to be responsible for and thus what they are 'forced' to do. Babies and toddlers will fight against having nappies changed, wearing socks, having their nails cut, being strapped into a car seat, not being allowed to touch knives or run into traffic, however fun it looks.
I'd say the acid test is probably how the majority of those children will feel retrospectively about your parenting of them when they are adults. There aren't many adults who feel trauma that their parents cleaned them up and didn't leave them to run around in their own poo, kept them safe in cars, prevented them from cutting themselves on knives or get electrocuted etc. - quite the opposite. I certainly haven't seen any MN threads asking how these things have adversely affected the rest of your life.
Essentially, these are all stages that an adult has to dictate on your behalf, before you are able to understand or take responsibility for yourself, with the goal of becoming an independent older child, teenager, adult, when they will no longer be necessary. They have an adult equivalent: wiping yourself after you've been to the toilet, putting a seatbelt on in a car, being careful how you hold and store knives etc.
By stark contrast, hitting other people is never something helpful or worthwhile that is needed in society - at any age. Ironically, for those who use hitting a person as punishment, there are various crimes whose enforcement will punish you FOR hitting other people.