I'm afraid I don't see Dawkins asserting (even implicitly) the immutability of physical laws as we know them. Nor do I think he saw it as part of his job in this essay to address the questions of the origin of physical laws or whether the physical laws we experience could be different. Why would you think he should have done that in this particular essay devoted to introducing readers to some elementary facts about Darwinism and evolution, and also to referring readers to other articles in the magazine?
I see it as intellectual totalitarianism to publicly call out your intellectual opponents with ad hominems suggesting no sane person even listens to them when the issue which is really at point, what is the source for the basis of one's own theory, is utterly ignored. I didn't say he said what he said, I said he didn't say it and his utter silence doesn't make it go away.
Thanks for the reminder of Dawkins, whom I haven't read that I recall.
On your 'immutability of physical laws' corespondent; I reccommend some reading on 'the Science Wars'. From the 'left', Norman Levitt writes Prometheus Bedeviled. Paul R. Gross and Levitt edited The Flight From Science and Reason that introduced me to this genre that I have found very satisfying. Alan Sokal's essay Transgressing the Boundaries: Towards a Transformative Hermeneutics of Quantum Gravity is a bright spot in the war against unreason.