Yeah, it could have said "you may move her to the city and remove one cube." However, if it's cured you would remove all 3 instead. That's the only reason for the specification of "treat disease."
That's one. Another reason is that the "Treat Disease" action has no effect on Superbug cubes and this power doesn't allow you to remove them if any are present in that city (from a previous outbreak in a neighboring city).