Sometimes this is justified by having the negative effects of magic outweigh the positive possibilities. In Barbara Hambley's "Windrose Chronicles", wizards are precisely pledged not to interfere because of the terrible damage they can do. In "Discworld", the importance of wizards is that they actively do not do magic, because when wizards have access to sufficient "thaumaturgic energy", they develop many psychotic attributes and may eventually destroy the world. This may be a direct effect or the result of a miscast spell wreaking terrible havoc.