Hmm, interesting. Perhaps it is rooted in the European concept of a nation state, where political legitimacy derives from cultural identity?

Btw, don't Native Americans identify themselves by their ethnic heritage as well? Not to mention South Asia (Hindu, Punjabi) and the Middle East (Arab, Persian, Jew, etc.). From what little I know of African cultures, many seem to also use distinct ethnic identities (Tutsi, Hutu, Zulu, etc.).