If we reject God, we can't simply assume the reality of the continued identity of the self (or even its very existence), an objective basis for morality, a rational basis for science, the existence of free will, the reality of the external world, the very idea of objective truth, etc. We need to mount arguments and evidence in support of these ideas if we want to be able to have a right to such beliefs.
And Sam Harris thinks we're lying to ourselves if we believe that our wills are free. His arguments are not particularly interesting or new here (and to many not even convincing). Harris may have just written a concise little book on the subject, but he's no Nietzsche, who clinched the case against free will and the self even more concisely, in less than a paragraph:
A thought comes when ‘it’ wishes, and not when ‘I’ wish, so that it is a falsification of the facts of the case to say the subject ‘I’ is the condition of the predicate ‘think’. It thinks: but that this ‘it’ is precisely the famous old ‘ego’ is, to put it mildly, only a superstition, an assertion, and assuredly not an ‘immediate certainty’. . . . Even the ‘it’ contains an interpretation of the process, and does not belong to the process itself. One infers here according to the grammatical habit: ‘thinking is an activity; every activity requires an agent; consequently —’.But where Harris is interesting (and I've subscribed to this line of thinking for at least a decade now) is in what he has to say about the implications of the denial of free will: it doesn't de-humanize us. This recognition humanizes us because it helps us to understand that instead of jumping to conclusions and throwing blame around, as we're wont to do, maybe we need to be more compassionate and understand that people are not fully free, and that their actions are at least partly to blame on circumstances and other causal antecedents...
While I agree with a good number of points made by Harris, there is at least one fundamental point on which he seems to be utterly confused: his denial of free will cannot be a scientific conclusion when he argues that there is no possible world in which free will could, even in principle, exist. If this is not a testable claim that could be decided by empirical evidence but simply by conceptual analysis (as I would be perfectly happy to do), then this is a philosophical conclusion... and people say philosophy doesn't make progress :)