This is the way the film-makers mindset works:
Calling a black man a nigger is OK as long as the man doing it is black. That way you can either explain it as "ironic" or clam that in a "blaxploitation" film it simply proves how badly black people have been treated by the whites over the years, to a point that they have been brainwahed to even insult their own.
Calling a dog nigger, equates a black man with a dog and so is totally unacceptable.
There was a an old kids film on which a dog was called Bender. Wonder if there would be a problem if they remade that now?