They lived during a time when summoning demons was done regularly, and required a little of the magician's lifeblood to keep a horde of creatures under their control. When a DemonMaster died, those bound under their control broke free. Because of this, many DemonMasters were hesitant to bind a demon permanently, since, upon their death, demons tended to go after their families; hence, DemonMasters made it so that demons were banished upon the magician's death.
By the time of Schooled in Magic, demon summoning is strictly forbidden throughout the Allied Lands. Most magicians frown upon demonology, since demons are living entities with wills of their own, and even the most skilled and careful magician can still wind up dead.