Actually, we DO expect more from the left. When an extremist compares Bush to Hitler, for instance, if other liberals do not publicly decry the remark, we assume they agree.
Frankly, it never occurred to me that another liberal might publicly decry such a remark. Similarly I can't imagine why any conservative should feel obliged to decry the comment of another. I figure, let people speak for themselves, and let the chips fall where they may.
This business of denouncing people, or hounding them to apologize or recant or disavow their own statements or actions is a form of totalitarianism akin to the Inquisition, or the show trials of Stalin.
No surprise then that it surfaced like a venereal disease under the influence of the 'rat-sucking media against GW Bush a couple years ago. Since then it has spread like an epidemic of genital warts and not even good conservatives have been immune.