Why is Maria so upset when Bruno keeps complaining that his father made a terrible mistake in coming to Out-With?

Bruno's father has been very good to Maria and her family, thus, she is appreciative for all he has done and loyal as well. This doesn't, however, help her understand why he is working at the camp. She believes him to be a good man, which is why his work at "Out-With" makes no sense to her. Note, she does not directly say this to Bruno.... she only alludes.