They promised the fans a faithful adaptation of the books though. They did not promise a reimagination. The backlash is on them and they put this actor in the firing line. By changing the race of Snape they cannot keep his story true to the books as they promised. A black boy growing up in the 70s/80s, in England, who is in love with a white girl, being bullied and singled out by a group white boys. That adds a whole element to the story that did not exist before and it changed the way the audience feels about him and towards him. Being black is more than skin colour, it is about community and culture so to change Snape's skin colour and not change his culture makes no sense. Black Snape would have grown up in a black household and that would be reflected in his character and culture. Again, this is all on them don't promise fans faithful adaptations.