"Black people can't be racist" has been exclusively used (in my experience) in reponse to critisism of them throwing the word "nigger" around within their own group. "Black people can't be racist" is just shorthand for "A nigger can call another nigger a nigger, but a white man cannot call a nigger a nigger without it being racist".
It's like saying, "It's OK, I'm Jewish" after telling a joke.
And...the issue with that is?
Okay, you aren't part of the in-group. So what?
Here's the thing...I use the word "nigga" in informal settings pretty freely, if I know that others will accept it. I'd never use it around my mother, I absolutely never use it at work. Ta-Nehisi Coates points out that it's not much different than having someone you don't know walk up and call your spouse "honey". This argument is actually very simple. We all know what the boundaries are, and yes, they're based in part on your skin color, your sex, your orientation, your political affiliation, your family and friends. So, let's be clear - if you aren't part of the group, you don't get the benefit of the doubt. The end.
But more importantly, we can see how race actually affects people's lives. For example, this case. White dude with no shirt on shows up and begins shouting slurs at a pro-Palestinian demonstration. Then, he accosts a black guy who was simply walking into a nearby mall. A security worker shows up - and immediately pepper-sprays the black guy, and then hauls him off even as the protestors tell him that he's got the wrong guy. That's the sort of thing I personally worry about (I also worry for the many young women in my family, but those worries are often far different).