"Maybe the Admiral is kind enough to only take those who won't curse him for it?" Sebastian has some doubts about that, but until he knows more about the situation, it's probably unwise to air them. He's also aware that Martha was one of the wardens and probably subscribed to whatever philosophy helped run the place. Rationally, he knows that there are gaols and rehabilitation programs that are effective and respect the rights of their inmates, but his viewpoint has been skewed by repeated exposure to the cruelty and senselessness of Azkaban.