Why place limitations and rules upon yourself unnecessarily? I could be wrong, but with agreements such as the Geneva Convention, one nation breaking it could result in sanctions and condemnation from the other nations in the world, and perhaps the U.N. being mad at you. This is nothing comparable to the setting of the game, where there isn't any galactic council of various races or anything, no higher authority or collection of races/worlds to answer to, etc.
Further, the aliens are already being treated differently anyways, being cut open and studied when dead, etc., and they don't follow ethical rules either by going after civilians.
All things considered, it just doesn't make sense for humans to contrasensically limit themselves with self-imposed rules and give hostile aliens "rights."
"Rights" make sense when fighting one's own kind, such as humans vs. humans, but not against an alien threat that itself won't play by "rules."