They may have not been practising Muslims, or Muslims in the eyes of other Muslims, but it doesn't hide their background.
It is a cultural issue, where their awful treatment of white girls stems from their culture's lack of respect for women - especially western women who are perceived to be decadent.
Islamic countries for the most part have cultures of misogyny that transcend religious practice. This culture is being imported into the west and we're paying the price for it.