Why is preventive medicine exempted from ethical constraints?

It is a paradox that medical experimentation on individuals, whether patients or healthy volunteers, is now controlled by strict ethical guidelines, while no such protection exists for whole populations which are subjected to medical interventions in the name of preventive medicine or health promotion. As many such interventions are either of dubious benefit or of uncertain harm-benefit balance, such as mass screening for cancers or for risk factors associated with coronary heart disease, there is no justification for maintaining the ethical vacuum in which preventive medicine finds itself at present.