Acceptable to whom and upon what criteria? Self preservation. Fear. Psychological fear, horror, and cultural conditioning. If I'm a serial killer and I like torturing babies, you have no intrinsic moral template, valid not only for you but for the universe as a whole, by which to judge my antinomian morality. This is the categorical imperative. As long as torturing babies is fulfilling for me, in Dawkin's world, this is as far as we can go epistemologically. You may quite rightly incarcerate or kill me for so doing, but you cannot judge me, at least not morally.
One aside here I have to mention is that the entire secular humanist paradigm has, in actual practice, had precisely the effect upon society Beckwith implies is should. One of the best evidences of this is the practice, popular for my entire life time, of calling Adolf Hitler insane, as opposed to evil. This shibboleth has worked its way into the very fabric of our culture, such that people like Charles Manson or heinous mass murderers are routinely called insane because our epistemologically relativist culture has lost its language of moral outrage.
Ideas have consequences.