There's perhaps a distinction to be made between "natural" and "naturalistic". I would think of "natural" to mean that it attempts to look like what the animal would encounter in its wild habitat. "Naturalistic" would mean that it at least resembles or evokes things found in nature *somewhere* - so, substrate that looks like some sort of soil, leaves, bark, branches, rocks, etc; as opposed to newspaper, RBI hides and cardboard boxes, bright pink dog bowl, crashed star destroyer in the background.
I suppose "natural" could also just mean things like real branches, live plants, real rocks, etc, even if they don't come from the same place as ball pythons do - instead of "fauxliage" and aspen substrate.