That's a convenient method when the patient can also make a phone call / appointment to inform the doctor they don't seem to be working rather than extorting (and I'm not using that word lightly) $150+ a month. Also consider the multitudes of other medications where there is no real argument to require a monthly extortion fee.
It seems to be this is more for the benefit of the healthcare system financially rather than the patient or the health of the population as a whole.