Chex - fantastic post - athletes are not automatons (even the ones on PED's), and culture plays a large role in incenting people to focus on certain sports or activities - and the most talented and dedicated among them will end up shining.
I note that people are mentioning body weight, body type, etc., but this too relates to culture. Kenya and and Ethiopia still experience deep poverty and is not a place that is going to sprout legions of 200 pound teens stuffing Big Macs - a good thing at that, eh?
And I am not sure that any statement as to whether they train is dispositive of anything. If they train harder, well, its likely because they are capable of doing so - because they are for a whole host of reasons superior athletes (culture among them).
I prefer to just to appreciate the athletic prowess of the East Africans, and for the sake of the sport, irrespective of the national origin of the athletes, the drug problem is somehow greatly mitigated.