People volunteer for research studies for many reasons.
In any case, it is generally agreed that few people would volunteer if the study wasn't worth their time, effort, and sometimes their discomfort. Ethical payment to study subjects is supposed to be limited to reimbursing their expenses, mainly for travel to the research center and for their time in the study. It is the reimbursement for time that is the tricky part. Usually this modest level of reimbursement doesn't generate too much concern because it doesn't seem like a coercive amount, just an incentive. But for some populations, like cocaine addicts in recovery, the cash raises the possibility that it will be used to buy illicit drugs.