The problem with Robertson's thought experiment, I feel, isn't that it's extreme or visceral, but rather that it is strawmanning an overwhelming majority of atheists. (Scott actually coined a term for this sort of thing: weak man.)
Most atheists I know don't in fact believe that God is the only possible source of morality; in fact, many of them hold that even if God existed, they would still evaluate each of His commandments on their own merits before deciding to obey. The mere fact that you don't believe in God doesn't make you a moral nihilist all of a sudden. Robertson's thought experiment relies upon the implicit assumption that atheism implies moral nihilism, making it okay to rape and murder, which is frankly a very old argument that has been refuted a great many times, both on and off the Internet.
Nitpick: Scott didn't coin "weak man", he mentioned it because the term appeared in the fallacy/bias literature. Previous to that on Scott's blog, I coined the term "flesh man"; later someone proposed "tin man". I don't know why Scott didn't use my term :( , or "tin man", either of which is much better.
A flesh/tin man is an argument/position that a real person actually holds, but where this real person has been selected to represent the worst of whatever side/camp you want to tar with the brush of foolishness or nastiness.
Link to Blog Post: "Extremism in Thought Experiments is No Vice"
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This is a LW discussion post for Yvain's blog posts at Slate Star Codex, as per tog's suggestion:
Scott/Yvain's permission to repost on LW was granted (from facebook):