This faux outrage at name calling is invariably highly selective. Normally it is in favour of one of the "protected species" black, brown, islamic, female or LGBT people. Meanwhile straight, white, Christian males are fair game for anybody to disparage or name call.
The Urban dictionary for instance defines the term "Kuffar" thus: Also spelled 'kafir' or 'kaffir', Kuffar is a highly derogatory Arabic term used to refer to non-Muslims, though it is usually directed less against "People of the Book" (Christians and Jews) and more against others (Hindus, Buddhists, Shintoists, etc).
A word the equivalent to the "N" word, is totally OK because it is not white people using it.