And because there are straight-up racists out there, people on Twitter objected to the idea of Hermione as a black woman. Others rejected Dumezweni supposedly not because she's black, but because she isn't Emma Watson.
J.K. Rowling released her ruling immediately after the casting news:
Alongside the good old-fashioned Twitter outrage, some people were actively awaiting Emma Watson's response.
Seeing as how Twitterers were denouncing Dumezweni in the name of Watson fandom, the onus was on Watson to shut them down.
Dumezweni didn't need the blessing of the previous actress to play Hermione, but she definitely appreciated it.