It's the same mechanism whereby a store has that one beautiful display model in the window which is seventy bazillion dollars and they don't expect anyone to buy it. It makes the rest of the stuff look reasonably-priced by comparison.

Extremists can also alienate, but only if they're the sole visible voice. I notice that non-fundamentalist Christians are largely invisible: if they held a "Walk for Christian Sanity" or some such, perhaps then the extremists would function in the previously described way, but for now they're speaking for the entire religion. (Also, the Pope defending and shielding child molesters doesn't help matters much; when the acknowledged head of a group is advocating protecting criminals, it's hard to listen to the rank and file.)