Visitors need to take the drug.
Hard to find info beyond visitors, but
this article does mention that some level of immunity is acquired. But it still claims that 90% of malaria deaths occur in those regions and that locals are indeed susceptible to the disease.
In all malaria-endemic countries in Africa, 25–40% (average 30%) of all outpatient clinic visits are for malaria (with most diagnosis made clinically). In these same countries, between 20% and 50% of all hospital admissions are a consequence of malaria (see country profiles for details). With high case-fatality rates due to late presentation, inadequate management, and unavailability or stock-outs of effective drugs, malaria is also a major contributor to deaths among hospital inpatients .
On reading this, I'm less convinced that locals are generally immune and untreated for malaria while only visitors are.