Hey. I think the reason is because of life points check. During Preparation phase you restore life points, so you need to make Cemetery check before doing that based on life points you were left after previous scenario. Without this specific wording you would be able to restore life points and then use Cemetery effect.
And yes, you can use Cemetery before your first scenario if it is already built before scenario Preparation step and your sum of life points left after Introductory scenario was more than 10.
