Came here to see if there was a post on this already, after reading
this news item today. When the guy says "Why don't you guys learn about history, the Emancipation Proclamation Act? You're only free because of our forefathers." - that's just an insane view to hold. So now black people should be grateful to the US for having abolished slavery at some point...?!
These are really awesome posts! I would add, though, that this kind of whataboutism happens very regularly to groups with leftist claims. (Maybe also others, but this is what I notice.) 'If you care about animals, why don't you also care about the working poor?' Replace 'animals' with any cause and 'working poor' with any unrelated other cause. (Also: 'If you're Raheem Sterling, why don't you always comment on any racism happening anywhere?' But that's back in the context you were discussing already.) Of course, many people are defending a cause on behalf of the cause, without being involved themselves. (E.g., animal activists are not animals themselves.) In that sense, it's definitely more nefarious for black people, as they are not even allowed to just talk about their own issues, while the suffering and injustice they have experienced eclipses pretty much anything - as you point out. But I do think it's part a more general reflex of anyone who disagrees with whatever instance of activism.
Maybe not really what you were saying, but this reminded me: I have often heard that being colourblind to skin colours is not necessarily a good thing. It's probably helpful in your own behaviour towards others, but it may cause you to miss that some people are being discriminated against due to their skin colour, because it's not a factor you consider yourself. So we are now thinking that we should not raise our children colourblind, but rather very aware of skin colour and discrimination, so they can actively battle it. (If only through their own behaviour and interactions - but all the same.)