Thanks for clarifying, I see where you're coming from now.
Between blackface and gangsta rap, there is a difference in kind: Black face is emblematic of society itself continuing the practice of dehumanizing a group of people that historically were dehumanized and stripped of all freedom. Gangsta rap represents something that is not nearly as systemic.
That's somewhat beside the point though. You touch on a relevant question: Even if many fewer people are offended, they are still offended. So the question is, should society not urge any action at all for offensive speech? Should no one feel any pressure to modify their behavior in some way? If you don't want to go to those extremes, then you have to draw the line somewhere. Currently society draws the line at racist content, specifically that aimed at traditionally downtrodden minority groups, and even more specifically, those minority groups that still experience significant bias.
Why does society draw the line where it does? My guess is because the scars there are not completely healed over, even if society has "recognized the error of its way". On other stereotypes we aren't as adamant. Irish, for example. We don't view the drunken Irishman stereotype with quite the same shock, probably in part because the institutionalized oppression (In the US) has been mostly dead and buried for a 100 years. In the UK & Australia however, the wounds are more fresh, and reference to derogatory Irish stereotypes are much more taboo.