" Why do you think they call them "witch doctors"? "

And who would be the "they" referred to? Certainly not the African people.

"In the genre of African colonial literature, the ‘witch doctor’ — Gagool in Rider Haggard's King Solomon's Mines, for example — serves as an allegory for the dark, sinister, forces of barbarism.

This is a gross distortion of the actual situation in Africa; where folk healers are highly respected, are linked to the benevolent ancestors, and work to ensure the well-being of their clients. In fact, the term ‘witch doctor’ is a misnomer. These functionaries neither doctor witches nor practice witchcraft, though they may occasionally act as witch-finders who smell out the secret workers of death and destruction. The term ‘folk healer’ is thus more appropriate.

In many parts of Africa the practice of folk healing incorporates two roles — that of the diviner and that of the herbalist."  

I. Niehaus