Let me concede that if you don't see a distinction between these two lives [the Good Life and the Bad Life examples given] that is worth valuing (premise 1 above), there may be nothing I can say that will attract you to my view of the moral landscape. Likewise, if you admit that these lives are different, and that one is surely better than the other, but you believe these differences have no lawful relationship to human behavior, societal conditions, or states of the brain (premise 2), then you will also fail to see the point of my argument.
The problem is that he presumes a "lawful relationship", he is begging the question. The issue isn't whether "an atheist can be a moral person". The question is whether morality exists, in any real way beyond preference and convention. While the presumptive answer of the book is yes, this is an implicit recognition that no, there isn't. A strictly materialist view of the world can tell us what we "ought" to do if we wish to achieve certain goals, but it is difficult to suggest it can tell us what we "must" do out of obligation especially in those instances where it comes at the expense of our own personal well-being or those of our loved-ones.
(I understand that their is a significant strain in Jewish thought supporting what we might call natural law, but I do not believe it can be asserted, at least not automatically, that this position would be maintained even in the hypothetical absence of the Creator.)