Regarding chalking things down to "personal choice," the pool of scientists is large enough and the pool of permanent staff positions small enough that there will always be scientists willing to make the "personal choice" to do weapons research. That's the thing about "personal choices," someone somewhere, under the right conditions will make the "personal choice" to do something. If you want people to stop making bad "personal choices," change the underlying situation or the politics.
Societies and the world do not run on personal choices. Sometimes personal choices matter and can have reverberations throughout society, but assuming society can function on an accumulation of personal choices is a child's way of viewing the world. If you want to change something you find disagreeable, influence policy makers or somehow figure out how to change the academic landscape in America (which probably means you again need to influence policy makers). Otherwise, you'll keep finding more scientists making the "personal choice" to develop weapons for uncle sam.
I fail to see how the fact that other people might be willing to do this research changes the ethical propositions at play. You are responsible for your research! Especially when you know what it is going to be used for. Refusing to do the research yourself might not stop it, but you refusing loudly, and convincing a couple people around you to not do so, adds up over time!
You are of course correct that the best thing is to institutionally change things. But the institutional changes are usually pushed for due to actual public pressure.
Societies and the world do not run on personal choices. Sometimes personal choices matter and can have reverberations throughout society, but assuming society can function on an accumulation of personal choices is a child's way of viewing the world. If you want to change something you find disagreeable, influence policy makers or somehow figure out how to change the academic landscape in America (which probably means you again need to influence policy makers). Otherwise, you'll keep finding more scientists making the "personal choice" to develop weapons for uncle sam.