Why can the unpardoned not object to their punishment?
The unpardoned cannot object because they are guilty and have been sentenced, receiving exactly what they deserve. As hell-bound sinners, they are just as deserving of God's punishment as anyone. Their punishment is just, so they have no valid basis to protest.
Les explains that the unpardoned are like the other seven in the analogy: they were guilty, they were sentenced, and they are going to what they deserve. They cannot say a word in objection because their punishment is just. In contrast, believers were also hell-bound sinners just like everybody else, equally deserving of God's punishment, but they escaped because they accepted God's pardon by His grace. So the unpardoned have no grounds to object; their punishment is deserved.