Sure it is. My first job outta college was for a defense contractor. It's perfectly fair to ask me why I left, or even more pointedly, "how do you feel about working for a defense contractor?"
It's not an invitation to badmouth, it IS an invitation to discuss the ethical concerns involved, and why you found it acceptable (even if it's just that it was a meal ticket until you found something you objected to less)
It's not an invitation to badmouth, it IS an invitation to discuss the ethical concerns involved, and why you found it acceptable (even if it's just that it was a meal ticket until you found something you objected to less)