My experience says that your knowledgeable posts about 3-year olds fit in the set of topics above.
Again, you can believe what you want.. since it makes your world view more stable for you, but I deal with 3 year olds all the time have for a decade now, not counting siblings growing up, and there is absolutely no doubt in any of my experience where fault lies for 3 year olds who believe temper tantrums are EVER ok.. and that is squarely and completely on the ineptitude of their parents.
A temper tantrum is something a child old enough to communicate should happen only 1 time, 2 if the child is a slow learner. Of course if parents handle it poorly it becomes a regular and routine occurrance, and of course like you the write it off as normal, because heaven forbid they actually realize their offspring is being a brat, be the parent and put a stop to it.