From the standpoint of instrumental reason, belief-formation is but one activity among others: to the extent that we have reasons for engaging in it, or for doing it one way rather than another, these are at bottom a matter of its contribution to our ends. What it would be rational for an individual to believe on the basis of a given experience will vary not only with respect to his other beliefs, but also with respect to what he desires. From this it follows that no amount of mere argumentation or experience could force one on pain of irrationality to accept even the factual claims of empirical science... Unfortunately for the contrast Ayer wished to make, we find that argument is possible on scientific questions only if some system of values is presupposed.
This need not imply epistemic relativism, since "epistemic warrant may be tied to an external criterion - as it is for example by causal or reliabilist theories of knowledge." (p.171) Still, on this account we cannot say that creationists are irrational. They merely fail to adhere to the objective, external norms we would hold them to -- same as the fascists. In either case, we (generally) have plenty of good reasons to care about those external norms. So what grounds are there for thinking the rational status of ethics and science differ?