Because we're not talking about the ethics of the individuals concerned, we're talking about the ethics of the people conducting the study. People volunteering

Because we're not talking about the ethics of the individuals concerned, we're talking about the ethics of the people conducting the study. People volunteering for this and the army have made the choice to put others ahead of themselves, and (as long as they are suitably informed and competent to make the judgement) have the right to make it. The ethics from the other side are broader and subtler - they are not of the individual they are of the masses. Whether the Army acts ethically by recruiting soldiers and sending them to kill and die is a subject of much debate. A large number of people think that it is unethical, and that is why they protest against war.