essentially because the emperor is a minor, and under guardianship of the duke, his education is also under his jurisdiction. alicia doing this is basically against the law, so even though they are keeping him in the dark, legally they are in the right, but its a stupid move considering how much the emperor cherishes his sis. and it makes the original story that much more enraging, as it means edith purposefully and successfully both cultivated a child into a psychopath for the purpose of getting ares and/or the crown