You don't need to be black to write a black person. You don't need to be female to write a female. Sure, it can help in certain cases, but to suggest that certain ethnicities and genders should ONLY write within their boundaries is in itself racist and sexist.
That said, Marvel is still doing it wrong. They are completely heavy handed with their pandering, to the point where it comes off feeling like a white jewish man in shining armor stepping in to defend the honor of all black people and all women.
Bendis thinks because Miles is Spider-Man, he should represent all "people of color" (I fucking hate this term) simply because he's black. Nevermind that Spider-Man has been considered a pest, a nuisance, and at times a danger to all of New York. The important thing is that he's black, guys! People of color, guys! Diversity, guys!