Just off the top of my head; the existence of 50 gm+/day licenses come to mind. Especially if the licensee in question grows trees in a DWC rig in their home. Granted, most would not, but you always have an outlier and there's the insurance industry's justification.
As to other house plants, have you thoroughly read your policy lately? It seems that more and more exclusions are added at each renewal as far as water or mould damage is concerned; especially damage that occurs over time rather than from a single incident.