I guess I don't see how an Islamic woman choosing to wear a headscarf is any more an endorsement, tacit or otherwise, of those factions of Islam that opress women, than a Christian or Jewish woman who choses to dress modestly is endorsing the oppression that has existed, and still exists, in certain factions of those faiths.

Face it, most cultures have some standards of modesty, and these usually have some connection to the religious traditions of the culture. That doesn't mean that anyone following the cultural norms endorses all the excesses of those traditions.