Fiddly, I hear you - it can be difficult to comprehend how these situations can arise if you've never been in them. Before I had DS I would have said exactly the same - if you don't have experience of a child like the ones we're describing, it's hard to understand that some children are so, so resistant to parental influence.
You say;
Kids throwing themselves on supermarket floors etc is just nonsense. Let them do it once, follow it through, abandon your shopping, put that child over your shoulder if need be and leave the shop. Straight home, into bedroom, see it through, don't give in until tantrum stops. Talk it through, let them say sorry and they won't do it again. It didn't get the required result.
I have a few questions/observations for you;
What if this child was 5/6? How long do you let them scream for? How physical do you let them get? How would you throw them over your shoulder whilst also trying to push a buggy with a toddler, negotiating them across the car park trying desperately to hold on whilst the elder child tries to run away? And how on earth, at that age, do you propose to wrestle them into their car seat? And even if you manage it, how do you deal with them popping the seat buckle every time you clip it in? How do you deal with the younger child seeing you be slapped and pinched and punched as you try?
And then once you're home, how do you manage getting both kids out of the car safely and across a busy road, unlocking the door whilst still trying to keep both kids safe and stop the toddler from wandering into the road and the 5 year old from running away? And once you're in, how do you respond to the older child taking her shoes off and throwing them at your head, followed by every single thing she can get her hands on. You put her in her room, fine, but she comes straight out again. By this time the toddler is screaming and the things being thrown at you are barely missing him. How do you keep him safe whilst continuing to put your elder child back in her room (repeating it maybe 50-100 times)? Oh and what about the friends that are supposed to be coming over for lunch in 30 minutes?
And, most importantly, how do you manage all of this when the professionals working with you have warned you that physically preventing your child from attacking you in this way (ie. holding hands or pinning arms) may be construed as assault and result in social services involvement? (Yes, really)
And how many times would you be willing to go through with the above? How many family birthdays, days out, meals with friends, park trips, would you be willing to sacrifice? And how much fear and stress and rage are you willing to live with, day in day out? And how willing are you to let your younger children grow up in an environment of constant shouting and screaming and tussling? And how long will you let it carry on for before acknowledging that your approach isn't actually influencing your child and making a difference in the slightest to their choices.
Some children respond to the approach you've described after a few occasions. Great. But we are talking, for the most part, about kids with control issues for whom being 'in charge' trumps everything. Kids who regularly provoke their parents to engage in distressing and fruitless power struggles that look to control the dynamic of the whole family. And you seem to imply that refusing to engage in these power struggles is incompatible with consistency ("everyone knows consistency works but it doesn't come natural to some people") and boundaries ("Kids without boundaries are confused").
This is absolutely untrue. I am teaching my kids that their parents are in control and in charge. We write the rule-book, and because of that we don't need to prove ourselves by responding to every invitation to a power struggle that they issue us. But precisely because I'm in charge, I can ignore the provocation in the moment, wait for them to self-regulate and issue a significant consequence when I see fit.
I appreciate that you don't fully understand the kind of behaviours and attitudes we're talking about here. But it would be a kindness to take our word for it that we are not parents without boundaries.