Why is an African American considered different because he/she talks proper?
I assume that you are saying here that African Americans who speak properly (“talk proper” is not actually grammatically correct), are seen as different by their fellow African Americans. In other words, you are saying that black people do not like it when other blacks speak properly. There is some dispute as to whether this is objectively true. However, I will assume that it is true for the purposes of this answer.
To the extent that African Americans think in this way, it is because they have been looked down upon by white society ever since the end of slavery. Many whites have believed (and do still believe) that blacks are in some way inferior. They may no longer see blacks as genetically inferior, but they still tend to think that black culture is problematic in some way. Even if whites do not look down on blacks, everyone knows that whites have more power and higher status than blacks in this country. Because of this, blacks tend to react to white culture in an insular and defensive way.
Because blacks are of lower status in the US, they naturally want to do things that will make them feel good about their race. One thing that they can do to achieve this is to feel racial solidarity and pride. They can feel that their own culture is special, unique, and valuable. This way, they can feel good even though they do not have high status in society.
Because African Americans feel this way, they often react badly to fellow blacks who seem to be rejecting black ways. If African Americans reject their own culture, it weakens the feeling that the culture is valuable and that it should be a source of pride. This is the main reason why African Americans object to black people who speak “proper English.” They feel that those African Americans are rejecting their culture, thus making it harder for African Americans to feel pride in a society in which they have low status.