This provides an eloquent answer both to "It's their culture" and to "Where are the moderate Muslims?"

As for "cultural relativism", I think this term covers several different ideas:

1. The methodological relativism without which anthropological fieldwork simply cannot be done. One has to recognise that even repellent beliefs and practices make sense to the people who believe and practise them, and that they do so by fitting into the cultural schema through which your informants view the world, said cultural schema being your (the anthropologist's) subject of study.
2. The realization that just about everything you've grown up thinking of simply as "the way things are" is another such cultural schema.
3. The false deduction that, since we cannot know anything without a cultural schema, everything we know is nothing but a cultural schema. And that, therefore, the only standard by which to judge a belief or a practice is its fit into its believers' or practitioners' cultural schema.

In my opinion, recognising (1) and (2) is necessary to becoming an enlightened person in today's world; (3) will therefore always be present as a pitfall to avoid.