To which civ did Taoism spread in North Africa / Europe? If it was the independents / barbarians, your complaint holds some ground, because they have rather high spread figures for Taoism (I guess Rhye made it so because they can be anywhere on the map).
In any other case, the European and African civs already have very low Taoism values. This doesn't completely exclude the possibility of Taoism spreading to a city without religion though - it's how the game works.
Crosspost edit:
I agree, but the situation was that Mali and Turkey went Catholic so often that it in the end resulted in the situation you describe. Turkey should become Muslim most often because else there's really a lack of conflict, and I hope the fact that Mali only gets one missionary still leaves a chance of them becoming Catholic (most of their trade routes go to Europe once discovered).