However, Kirzner seems to me to be charging that Rothbard ignored the fact that Austrian Economics, itself, was a value free science. This, surely, was not the case.
In The Ethics of Liberty, Rothbard discusses the views of Mises on this very topic:
Mises attempts to reconcile his passionate advocacy of laissez faire with the absolute value freedom of the scientist. This is to take a position much more compatible with praxeology: by recognizing that the economist qua economist can only trace chains of cause and effect and may not engage in value judgments or advocate public policy.Rothbard, as Kirzner contends, did not ignore the value free nature of economics as a science, rather he used Austrian economics as a tool to go beyond economics and apply its findings to understanding
This route of Mises concedes that the economic scientist cannot advocate laissez faire, but then adds that he as a citizen can do so...
Thus, while praxeological economic theory is extremely useful for providing data and knowledge for framing economic policy, it cannot be sufficient by itself to enable the economist to make any value pronouncements or to advocate any public policy whatsoever. More specifically, Ludwig von Mises to the contrary notwithstanding, neither praxeological economics nor Mises's utilitarian liberalism is sufficient to make the case for laissez faire and the free-market economy.
To make such a case, one must go beyond economics and utilitarianism to establish an objective ethics which affirms the overriding value of liberty, and morally condemns all forms of statism, from egalitarianism to "the murder of redheads," as well as such goals as the lust for power and the satisfaction of envy.
the value of a libertarian society.
Rothbard wasn't improperly merging value free Austrian theory with libertarianism, he was using Austrian theory to understand libertarianism. This is an important difference. In the former, the value free nature of economics is ignored, in the latter economics is used to help understand and explain the nature of a libertarian society. Two very different things.