Consider the following Marxist argument:

1. Economic freedom and private property in the means of production immiserate almost everyone.
2. If some economic regime X immiserates almost everyone, then, pro tanto, X is morally bad.
3. Therefore, pro tanto, economic freedom and private property in the means of production are morally bad

Libertarians deny 1, as they should. Marxist economics is bad economics. Few professional economists are Marxists. Most people who accept Marxist economics are non-economists, and they do so, as far as I can tell, largely because Marxist economics flatters their ideologies.

Many libertarians also deny 2. Perhaps they think they need to deny 2 to protect their ideology. But do they? This is one place where I differ from hard libertarians and why I am more of classical liberal. Premise 2 seems pretty plausible to me. (Note the “pro tanto” qualifier in the premise, as well.) When hard libertarians deny premise 2, they come across to others as free market zealots–“Capitalism, though the sky falls!”.

UPDATE:

Think of it this way.

Both the hard libertarian and I both say, “The Marxists are mistaken–free market capitalism does not immiserate the poor.”

The hard libertarian adds, “But if it did, I’d be in favor of free market capitalism anyways.”

I add, “But if it did, I’d want to reject free market capitalism, provided there were a better functioning alternative, and provided this alternative didn’t violate other moral principles.”