Once missions are launched, all land automatically becomes public. This allows any Explorer to play minigames or perform missions on your land, and once revenue sharing is released, will allow you to swap some of your CLNY generated daily for some of the rewards Explorers earn on their missions.
If you have enough avis to play all the missions on your plots, and don't want to participate in revenue sharing, you can make your land private to opt-out. This is reversible at any time.