Ethics study is rarely about what decision to make, but mostly about how to make a decision. So, there's no problem with teaching ethics in the presence of defense industry.
Similarly, doctors learn medical ethics, and, of course, not every question has the "right" answer. Partially, medical (and research) ethics are about knowing what constitutes malpractice under current law, but it's also about some more general ideas (on which the law might be based) that are hard to quantify. Here's one example: during a drug research, if the interim results show that the newly suggested treatment is unambiguously better than the one given to the control group, the researcher is compelled to stop the research and just move everyone to the new drug. But, the reality is rarely so clear-cut. The researcher might not be confident in the accuracy of the intermediate results. While the average success from a particular treatment might improve, it might also worsen the situation for some outliers in the target group etc. All this would lead the researcher to the situation where they need to select between continuing and stopping the research with no clear best choice.
Similarly, doctors learn medical ethics, and, of course, not every question has the "right" answer. Partially, medical (and research) ethics are about knowing what constitutes malpractice under current law, but it's also about some more general ideas (on which the law might be based) that are hard to quantify. Here's one example: during a drug research, if the interim results show that the newly suggested treatment is unambiguously better than the one given to the control group, the researcher is compelled to stop the research and just move everyone to the new drug. But, the reality is rarely so clear-cut. The researcher might not be confident in the accuracy of the intermediate results. While the average success from a particular treatment might improve, it might also worsen the situation for some outliers in the target group etc. All this would lead the researcher to the situation where they need to select between continuing and stopping the research with no clear best choice.