Technically, Frog and Robo aren't their names. Frog's true name is Glenn, and Robo's true name is R-66Y, or, as you learn later, Prometheus. Robo got his name because Marle didn't want to call him something as akward as R-66Y the whole time, and Glenn got his sometime in the Middle Ages (had he been named by your group, the people in the Middle Ages would have referred to him by his true name, which only happens on a few rare occasions), and there are plenty of ways to explain his name change. Since you're allowed to name them whatever you want anyway, I don't think there's much reason to complain about their default names.