Why does Fusion not allow safety stocks to be held against FG items?

Our environment is typically Make To Order with B2B flag enabled. However, for certain customers we however want to hold a safety stock of finished goods, to support contractual obligations. We were told by our SI this is not possible for FG & have been trying to manual manage this requirement (spreadsheets). However, this is proving to be difficult and we would like to understand more about a system solution.