Some people seem to think all slang terms for cultural groups are automatically derogatory. A sizable proportion of people from the US get offended that people from Latin America call them “gringos” too (which is ridiculous IMO).
"Ami" isn't just any slang term, it is a politically loaded word. Besides being used as a common derogatory expression it was often used by the RAF[1] and their followers, especially in the slogan "Ami – go home!".