Islamic law is well-defined through a tradition of jurisprudence. There is no mandate at all for female circumcision, however. Permitting such a ritual constitutes an act of tolerance by Islamic law for pre-Islamic practices, and may be overruled by the Islamic prohibition against harmful acts. Consider, for example, that Islamic law protects a woman's right to sexual enjoyment, as demonstrated by the fact that a woman has the right to divorce on the grounds that her husband does not provide sexual satisfaction.
People often confuse traditions rooted in local culture with religious requirements.
Ethiopians in the United States stand between the Ethiopian culture of their heritage and the American culture of their environment. They cannot and should not be expected to abandon their religion. I do think, however, that the young amongst them, at least, will be willing to abandon old-world cultural practices at odds with their adopted culture when such practices are unsupported by religion.