You're absolutely right in your characterization of things, but this is what a lot of people fail to get. What we have here are two separate moral standards going on.
Human beings have lived most of their existence in groups of no more than 150 individuals. Even for most of recorded history, most people lived in villages or in neighborhoods in cities where they knew just about every face they saw during the day, every day of their lives. Whatever kind of innate moral sense we have and whatever moral codes we have developed have all developed within this context of face-to-face interactions and persistent relationships. So, human beings have a hard time doing anything that isn't "nice." It's not "nice" to quit without giving notice. What "decent" person does a thing like that?
Companies, by contrast, operate on a system of profit and loss. I am not saying that's a bad thing. What I'm saying is that people shouldn't kid themselves. When a company decides to show you the door, that's excused as being "nothing personal, just business." In other words, they are doing solely what is the interest of the company: most particularly, their bottom line.
People need to understand that these are the rules. By all means, when you're interacting with friends, family, neighbors, or even strangers on the subway, do the right thing—the thing that human interactions have relied on for millennia. But when you're dealing with a company—when it's business—think first what's in your best interest, and then do that without a qualm.
Maybe giving notice is right for you, then and there. Then, go ahead. But, maybe walking right out the door is the best thing for you. In that case then, by all means, don't let the door hit you in the ass.