The following quotes from the REG should clarify. I highlighted the most pertinent point for your question, but the rest flows with the rationale:
Instant Abilities -> Discard or Remove -> Default Conditions
• If a site, artifact, or character is discarded or returned to hand, discard all cards placed on it. If a card leaves the Land of Bondage, discard all evil cards placed on it other than Lost Souls or captured characters.
• The following is understood when discarding a site: “If site still holds a Lost Soul, Lost Soul is returned to general Land of Bondage. Discard any other cards in that site.” Unless stated otherwise, holder chooses which site (occupied or unoccupied) to discard.
