You know, maybe this should be canon. The reason Diana wears an American flag is because SHE inspired the damn flag! Yeah, that's right, bitches, Diana came first. BECAUSE SHE'S WONDER WOMAN. America got all it's good ideas from her, because we're too stupid to come up with their own.
Also:
"What odd looking people!" "Yes, one is a Negro who is not in chains! And that woman is scandalously uncovered! Ah do believe I have the vapors!"