One being my sister - who is an excellent mom if I do say so myself. If either my niece or nephew started to throw a conniption, she'd leave whatever she was doing, shopping, whatever, and take them home. They learned pretty quickly - if I misbehave in public, we go home and I don't have any fun.
Perfect.

Ok here I come with my behaviorism hat. Yes, our good friends BF Skinner, Ivan Pavlov, Watson, etc. All behavior is learned, period, through various forms of reinforcement and punishment. That reinforcement can be subtle, to the point the parent doesn't even realize what they're doing. When you're dealing with children, they don't have the cognitive structures in place that adults do, so yes "kids will be kids." That also makes them very black and white in their thinking. But they learn from parents, watching other kids get away or not get away with things, other parents, teachers, etc. how to act. A baby doesn't come into the world with these bad behaviors, so they come from somewhere. There are some genetic contributions, but it comes down to environment as to how those genetics are shaped.

I don't watch "Nanny 9-11" or whatever the heck it's called. But one day it was on and before I could change channels I saw the intro. The kids were crazy and yelling and mom was yelling and screaming right at them. Where did the kids learn that yelling and screaming was acceptable? Probably from mom. But she certainly was maintaining it. When it gets that bad, it becomes very difficult to get to the point the behavior stops. There's a little thing called an "extinction burst" where the kid (or adult for that matter) increases the intensity/duration/frequency of the behavior you're trying to stop. So the tantrums become exponentially worse for example. If mom or dad caves, the child learns "ok I need to have a complete meltdown to get what I want." Start the cycle again to try to stop the behavior, get to the extinction burst which will probably be even worse than the first one. Mom or dad caves....rinse and repeat. It's that extinction burst where a lot of parents can't take it, because it's really bad a lot of the time. While I can sit here and be scientific about it, I can certainly see why you'd want to give in.

Of course I've simplified humans down to behaviorism theory. But underneath all the layers of complexity of our lives, this is what is the cause of behavior, or lack of behavior. In my opinion anyway.