'non-cannibals' is formed in English from the prefix 'non-' + 'cannibal', where 'non-' meant 'not' and 'cannibal' referred to someone who eats members of their own species.

'cannibal' entered English in the early 16th century from Spanish 'caníbal' (from Columbus's accounts 'Caníbales') referring to the Carib people, and came to mean an eater of human flesh; 'non-' as a negative prefix has been combined with nouns in English to form opposites (e.g., non-smoker), producing 'non-cannibal' and its plural 'non-cannibals'.

Initially, 'cannibal' was used (via Spanish) to refer to certain indigenous peoples and then to people who ate human flesh; 'non-cannibal' simply meant 'not a cannibal' and has kept that basic sense as 'does not engage in cannibalism'.