Forced selection for the sickle cell trait is not natural. The premise is false, as is the conclusion.
ANY "selection pressure" is "forced" in some fashion or other. That "force" may be a mountain range, continental drift, a shifting river course, or human intervention. You seem to be hung up on the fact that these groups were made slaves and lost their freedom...which is totally irrelevant to the question of evolutionary change. The "selection pressure" on Ashkenazi Jews was also "human imposed", and thus, by your lights "not natural". Mother Nature doesn't care one way or another.