: If free markets and capitalism are the natural expression of the human condition,
They're not natural. The "capitalist system" (whatever that means) is the product of thousands of years of human interaction. What we see presently can only be replaced and improved by piecemeal trial and error. The "abolishment of money and the nation-state" in one fell swoop as proposed by many here is simply a mystical flight of fancy. Actions such as these would spark immediate dissolution of even a pretence for behaving with even a modicum of certainty or continuity. Social cause and effect would be history and I speculate, opinion only of course, taht a vast majority of the world's population would wind up dead in a short time were this to happen.
: why must so much psychological, emotional, physical, political and economic force be expended to preserve it?
Funny, go talk to David Friedman and other political economy theorists and they'll give you a different perspective or what I like to call "an alternative voice":
The forces taht you speak of are tied to the realm of politics. Ya might want to do some brushing up on the subject of rent-extraction relating to the manner in which politicians basically extort contributions and other such considerations (called rents) from producers in order to abstain from screwing them legislatively. Your whole post seems tied to the gross misunderstanding of Marx's regarding the nature of social structures and institutions. He thought that republican government was merely the manner in which the bourgousie managed their class affairs. But such rent-seeking is obviously and demonstrably at odds with this theory.
As philosophers have said "the law that all swans are white is disproven by even one instance of a black swan". In the same manner, the "law of the classes" is disproven by even one instance where the supposed "class interest" cannot explain the underlying social dynamics. Rent-seeking is just one such example where "the law of class interest" simply does not contribute to meaningful analysis. The only way around this is to deny that laws are set and fixed and that they change with time and place. In taht case, why have law as anything anyone considers as law is, voila, law. I could probably draw some elaborate conclusion from the consumption of some randomly chosen commodity and then extrapolate a vague trend from it. But it would be as meaningless as is the naive classification of social classes based merely upon the "wage relation".
The foundationalist insanity of the materialist dialetic quickly leads us down the road to nihilism.