There is a concept in US intellectual property law called "life story rights". It's more commonly an issue with film and television, but it's also relevant to publishing.
Attorney Mark Litwak
writes about it here, with an emphasis on film and television issues.
Here's the thing. If the character of Aibilene is actually based on Ablene Cooper, and if
The Help includes stories based on events from Ms. Cooper's life (and this is what Ms. Cooper will have to document)--
--and if Ms. Cooper wanted to write her own book now, presenting her own story of her life as a maid during the Civil Rights struggle
--she may be forestalled from profiting from her own experiences and character because Ms. Stockett had beat her to the market.
Now, it's hard to prove these cases, but people have won them. And the idea makes sense, yes? People who are private citizens should have the right to tell their own stories, or to decide who gets to tell their story.
As for the "why did many black Americans dislike this book intensely?" question, some views are highlighted in this
New York Times piece.
For myself, as a white New Englander, I found Stockett's choice to render the black characters' dialogue in dialect and the white characters' dialogue in Standard English an awfully poor one.