In order for researchers to do human experimentation, they (in almost all cases) the research proposal must pass an ERB (ethical review board). This seems like a good idea, to prevent abuses.
More important than the ERB review is the consent form that all participants of the study get. In theory, it explains all of the risks, rewards, side effects, etc so that the person knows exactly what they are getting into when they agree to be part of the study. Problems can arise when people are willing to consent to something, but the ERB feels it is unethical. The first case of informed consent was with Yellow Fever. Healthy people were willing to get themselves infected with a disease was lethal about 30% of the time so that the disease could be better understood and fought. Such a thing would most probably not be possible today because of an ERB. Similar experiments that would be useful are things that would determine is possible carcinogens are actually carcinogenic, faster turnaround for drug development, and any disease about which not that much is known (or usually comes with complications).
If people are willing to participate in these kinds of studies, why should we stop them?