Simple answer - if the cream or lotion contains water of some kind it is a breeding ground for mould fungus or yeast and poses a health risk of infection therefore it must be preserved by some method. They are not required to be sterile but must be contaminate free. If the product is bone dry no preservative is needed.
But at the moment there is no TRULY NATURAL broad spectrum preservative available. The test data available is not conclusive and they have limited success compared to known broad spectrum preservatives which generally are engineered compounds or synthetics that retard bacterial growth. These are SAFER than bacteria.
Not all chemical compounds are dangerous just as not all natural products are safe eg nuts bee pollen honey can cause allergic reactions and cause death. Preservatives to avoid are those containing formaldehyde methyl parabens glycols and petrolatums.
So the final analysis is a balance between the peace of mind knowing the integrity of the product is maintained and keeping it as close to 'all natural' as possible.