You are right. Who are we to be judging other cultures? We are using our intolerant Eurocentric morality to judge another culture and their customs.
We would not expect them to judge our culture and insist we mutilate our children, so why do we think is is OK to be judgemental and intolerant toward their cultural practices? If we really wish to embrace diversity we must embrace without being judgemental. Each and every culture is morally equilivant to every other culture. We must not place emphasis on our own cultural values over any other cultures values. Multiculturalism is being non-judgemental, our strength is in our diversity.
In fact, Charles Manson and his followers were really a sub-culture within the larger culture. As such, they should be celebrated as a diverse and beautiful building block toward a stronger, more tolerant more non-judgemental America.