Correlation is not causation. One would assume a drug company would pay a doctor to oversee a clinical trial or enroll patients by looking at who are the most experienced ones ie. had the most patients (which means the most prescriptions). They would not be doing this sort of work in a market that had gone generic so those prescriptions would be branded.

So they look in a non-generic market for a doctor with lots of patients to enroll in their study and compensate them for their time. That would then show that 'drug company payments mirror brand name prescribing' not because the payment influences the prescribing but the prescribing is what allows the drug company to find the right patient population to complete the final trials to bring a drug to market.