To the extent that the cultural-appropriation police are urging their targets to respect others who are different, they are saying something that everyone needs to hear. But beyond that, they can plunge into doomed tangles. American popular culture is a mishmash of influences: British Isles, Eastern European, West African, and who knows what else. Cole Porter committed no wrong by borrowing from Jewish music; Elvis Presley enriched the world when he fused country-and-western with rhythm-and-blues.