In Swearing: A Social History of Foul Language, Oaths, and Profanity in English(1991), Geoffrey Hughes comments on the apparent friendly connotations the term carries in Black culture, and compares it to the variance between British English's use of the term 'bastard' as a horrible insult and Australian English's use of 'bastard' as a term of endearment. (32)

Later, Hughes remarks that "Mother-fucker appears to have developed initially in negro communities, proliferated in the Vietnam War and then spread across America socially and geographically with the returning troops." (page 201). It's a plausible argument if you think it through, but our readiness to accept it might rely a bit too heavily on pop-culture portrayals of Black culture. Unfortunately Hughes only makes the observation in passing and doesn't bother to cite anything to support it.

But if we accept Hughes' take on it, we can say that swear words like mother-fucker mean different things within different cultures, and the different uses eventually spread as more people become exposed to these alternate uses.