They weren't "exiled" by their own countrymen, they were sold into slavery by them.
In fairness tho', the African tribes weren't doing it on their own, the invading Arabs in the north of the continent had been slave-trading there since the first millennium....and still are today, thanks to obama's destruction of Libya
those same Arabs used N. Africa as a start off point for slave-raids on Europeans along the Mediterranean coast all the way around Spain, Portugal up to Britain and Ireland ....there are even remnants of these White slaves still living in the Caribbean today