One of the typical arguments trotted out against official apologies for historical atrocities (slavery, genocide, apartheid, etc.) is that "we" (that is, the folks still alive) weren't the ones who did the wrong. Indeed, often times we're not even descended from the wrongdoers. So why should we say we're sorry?
In his discussion about Australia's upcoming apology for "the lost generation" (where Aboriginal children were forcibly taken from their families on the grounds that the Aborigines were going to die out soon anyway), John Quiggin flagged the best response to that argument I've heard yet, from Raimond Gaita's book A Common Humanity. Simply put, "any moral position allowing pride in the achievements of our forebears and our community necessarily entails shame in their failings."
So, if I express pride in America's fight for freedom in WWII, or the emancipation proclamation, or the development of democratic political theory as part of my identity as an "American", then I similarly as part of that identity must express shame for the genocide of Native Americans, the enslavement of Black Africans, and the other myriad failings of American society. They're part and parcel of the same commitment I have as a member of this community. But too many people want the sweet without any of the sour.