That makes no sense at all.
One is an insult kept in the family. Blacks insulting each other is their own business.
The other is an insult aimed at someone else. A confederate flag is a symbol whites used to oppress blacks.
Or to put it another way. Suppose you had a sister. You probably wouldn't think anything of insulting her and treating her rudely. You're family and siblings tease each other. However if some stranger came up to her and started doing so, calling her a bitch and slut, thats ok?