To start with, their foundational article of faith, the "free market" that they bleat on and on about does not exist, never has existed, and never can exist - it's a theoretical ideal that assumes a perfectly frictionless market with perfectly *rational* actors (both buyers and sellers) who *always* act in their own best interest, and that there are no systemic factors that inhibit competition. It's a thought-experiment, not a description (or even a prescription) of the real world.
This is quite correct. The "free market" which advocates speak of is synonymous with perfect competition which, as Craig states, doesn't exist and actually can't exist. There will never be markets that have, for example, no barriers to entry, or an infinite number of buyers and sellers. It does however provide a theoretical yardstick to evaluate how real markets work, and what is required to satisfy the actual conditions (i.e., both positive and negative liberties). A very good introduction to the subject is the classic by Joan Robinson, The Economics of Imperfect Competition (1933). All the best, Lev